The speed of light varies in different mediums. In air, light travels about 186,000 miles per second. In a denser medium, such as glass, light travels about 124,000 Mps.

In 1666, Newton discovered that sunlight passed through a prism reveals the colors of the rainbow. The arrangement of the colors of the rainbow is called the color spectrum. A very close look at sunlight passed through a prism reveals that the color spectrum is not continuous, but is actually composed of discrete lines of color.

In 1913, Niels Bohr discovered that when you add energy to the atom, heat it, for example, electrons respond by jumping to a higher orbit. With each jump, each electron emits a photon of characteristic energy.

See UV below

COLOR RELATIONSHIPS

Primary colors used in painting are red, blue & yellow.  All colors are blended from these. (Computers use others; see below)	Complimentary colors are the colors that are directly across from each other on the color wheel.
Secondary colors are green, purple & orange.	Harmonious colors are the colors that are next to each other on the color wheel.  Like blue & purple.
Tertiary colors are the mixture of two secondary colors.	Tints are colors with white added.  Shades are colors with black added. A color tone is the degree to which it changes.
Colors are considered either warm (reds) or cool (blues).	Saturation is the amount of color used.  The more color, the more vibrant.  Pastels use only a hint of color.

Source: askthekitchendesigners.com/color.htm

Thomas Young (1773-1829) discovered that by combining just three of the seven colours¹; red, indigo and green; the result was white light. From these experiments, these three colours were considered primaries. He also realized that by combining any two of these three, gave you one of the other colours seen in the prism, or secondary colours.

Artist Michael Eugene Chevreul (1786-1889) is credited for formulating the first systematic attempt to study and write about the subject of color combining. From his laws of contrast and harmony have stemmed concepts of monochromatic, analogous, complementary and triadic color schemes that are found in most modern art textbooks, with Red, Yellow and Blue as primary colors.

Colours can affect what we think and feel: our moods. The reaction we have to a design -- anger, warmth, power, purity -- can all be bought into an image through the use of colour.

Black: The colour of authority and power.
White: Innocence and purity.
Red: Red is the most stimulating colour on the colour wheel. It can stimulate faster heart rate, influence rage and or anger, and can also stimulate appetite.
Blue: Blue invokes peace, tranquility and loyalty, but can also be cold and depressing.
Green: A difficult colour to master, yet it is one of the most relaxing (hence green rooms at TV stations). Dark green is masculine and conservative, and implies wealth.
Purple: is the colour of royalty, and gives the impression of luxury, wealth and sophistication.
Source: www.sitepoint.com/graphics/
See Also: Colour Energy & Healing

Color Space

RGB (red green blue) - Additive colors - displays
RGBA (red green blue transparency)
CMYK (cyan magenta yellow black) - Subtractive colors - ink
HSL (hue saturation lightness), HSB (hue saturation brightness) 
  HLS (hue lightness saturation),  HSV (hue saturation value)
SAS color names (from the SAS Registry)
Gray Scale
Pantone - Printer color table

RGB RGB - Red Green Blue - Amount of each color. Values from 0-255 (8-bit code).
  e.g. (255,0,0) = red; (255,255,255)= white; (255,255,0) Red + Green is Yellow; (127,127,127) is gray, (63,63,63) is light gray.
You will sometimes see it expressed as Hex or HTML codes with values from 00 to FF. FFFFFF is white.

See why do computers use different colors CYM, instead of the primaries we learned in school Red, Yellow and Blue?

  The visible spectrum is frequently broken down into seven hues (colors). These seven hues¹ are as follows: blue, green, red, cyan, magenta, yellow, and white or black.

The numbers e.g. 0,100,0 refer to the percent of Red, Green and Blue.

  Magenta (The purplish red color, also called fuchsia, which is part of the CYMK subtractive color model) is not found in the visible spectrum of light. Rather, it is physiologically and psychologically perceived as the mixture of red and blue light, with the absence of green. Red and blue, the two components of magenta, are at opposite ends of the visible spectrum and have very different wavelengths.

e.g. Black = (0,0,0), White = (255,255,255), Red = (255,0,0)

HSB (hue, saturation, and brightness) or HSL (hue, saturation, and lightness) also HLS.
It is generally agreed that color can be defined by three qualities: hue, saturation, and brightness (HSB)

• Hue
  

  Official Web Color Names
  black (0 0 0)	silver* (75 75 75)	gray (50 50 50)	white (100 100 100)
  maroon (50 0 0)	red (100 0 0)	purple (50 0 50)	magenta/ fuchsia (100 0 100)
  green (0 50 0)	lime* (0 100 0)	olive* (50 50 0)	yellow (100 100 0)
  navy (0 0 50)	blue (0 0 100)	teal* (0 50 50)	cyan/aqua (0 100 100)

  Note: Numbers above (e.g. 0 50 0) refer to the % of red, green & blue. e.g. pure red is "100 0 0"
  See New CSS 3 Colors at the Web Development page.
  Hue is the actual color or wavelength reflected by an object-red, yellow, green, and so forth. For example, it could be said that the color of an object is blue. Blue identifies the hue. See the color wheel above for systems based on subtractaive primaries.
  Hue is sometimes described as number of degrees, with red being 0°, green 120° and blue 240°.
• Saturation (Chroma) - The pureness of a color. In other words, whether it is only one or a few frequencies or many. An example of saturation is a red that was 'grayed out' by mixing in some green and blue. The saturation of a color is the degree to which the color departs from neutral gray of the same brightness. You can think of it as mixing black, gray, or white paint with a colored paint, thus diluting the color.
  

  Microsoft: Lo  Hi
       RGB %   50,50,50   75,25,25   100,0,0
  Photoshop: Lo  Hi
       RGB % 100,100,100 100,50,50   100,0,0
  

• Brightness/luminosity/lightness/Value (Tone) -The brightness of a color is independent of the hue. Two colors may have the same hue but different brightness. You can think of brightness as how dark or light a colored object would be if you took a black and white photo of it. Thus, to describe a color or brightness, we say that it is dull, bright, vivid, or brilliant.
  The word 'Tone' is sometimes used to mean a color value. For example, "That red hued text is very hard to read against that green hued background because the red and green are practically the same tone".
  Microsoft treats this parameter different than programs like CorelDraw and PhotoShop.

  Microsoft: Lightness  - Lo  Hi
           RGB %           0,0,0     100,0,0   100,100,100
  Photoshop: Brightness - Lo  Hi
           RGB %           0,0,0      50,0,0     100,0,0
              

Tint, Shade and Tone:
Every individual color on the Basic Color Wheel can be altered in three ways by Tinting, Shading or Toning.

Tint - Pastels - Created by adding white.

Shades - Created by adding black.

A Tone is created by adding both White and Black which is grey. Any color that is "greyed down" is considered a Tone.

See: The Difference Between a Hue, Tint, Shade and Tone

Lab
Lab mode has a lightness component (L) that can range from 0 to 100. In the color picker, the a component (green-red axis) and the b component (blue-yellow axis) can range from +128 to -128. In the Color palette, the a component and the b component can range from +120 to -120.

Tints, tones, and shades are variations of the hues found on the basic color wheel when white, black or both are mixed in. To illustrate this, I painted a Tint, Tone and Shade color wheel using Liquitex Basics acrylic paint for each of the 6 primary and secondary colors.
- Tint: The act of lighting a color by adding white to it.
- Shade: The act of darkening a color by adding black.
- Tone: Slightly darkening a color by adding gray.

Color Mixing

Additive Color Mixing

In video, the color spectrum contains three primary colors, namely red, green and blue. By combining these three, all the other colors of the spectrum (including white) can be produced.

Making colors in this way is based on blending, or adding up colored light, which is why it is called additive color mixing. Combining the three primary colors in specific ratios and known amounts enables us to produce all possible colors.

By combining the three primary colors red, green and blue, other colors can be mixed, including white.

White light is derived from a ratio of 30% red, 59% green, and 11% blue. This is also the ratio to which a color TV is set for black-and-white broadcasts. Shades of grey can be created by maintaining the ratio percentages and by varying the luminosity to specific values.

Subtractive color theory explains how we see objects which do not emit their own light, but rather reflect light that falls incident upon them. They absorb (subtract) certain colors and reflect what is left. In subtractive color theory the primary colors are: Cyan, Magenta, and Yellow.

Because these are subtractive colors, when the three are combined, they remove all color and result in black.

Subtractive Color Mixing

There are several sets of subtractive primaries:
Computer printers typically use Cyan, Yellow and Magenta. Black can be created by mixing all 3 but is usually created with a black ink.

	In painting red, blue and yellow are the primaries. Red + Blue = Purple Red + Yellow = Orange Yellow + Blue = Green Note: The blue (60% brightness) and green (40% brightness) here are darker than pure blue and green (which is called lime).
	CYMK=Cyan/Yellow/Magenta/Black. Yellow subtracts blue from magenta to make red and blue from cyan to make green Cyan subtracts red from magenta to make blue and red from yellow to make green Magenta subtracts green from cyan to make blue and green from yellow to make red

Why do computers use different colors CYMK (Cyan Yellow Magenta and Black) , instead of the primary colors we learned in school Red, Yellow and Blue?
 

The choice of primary colors is related to the physiology of the human eye, which has cones sensitive to red, green and blue.
In 1965 came experimental confirmation of a long expected result - there are three types of color-sensitive cones in the retina of the human eye, corresponding roughly to red, green, and blue sensitive detectors.
Source: RGB color model - Wikipedia

You're taught red, yellow and blue because they're CLOSE to magenta, cyan, and yellow, and they're some of the first color names you learn as a child.
Source: Why do computer screens etc use red, GREEN & blue? - rgb computers color | Ask MetaFilter
See also The Color-Sensitive Cones (gsu.edu)

See also: graphics - RGB for color composition rather than primary hues - Stack Overflow
Color Theory | cs.nyu.edu

Other Color Systems

L*a*b color is designed to be device independent, creating consistent color regardless of the device (such as a monitor, printer, computer, or scanner) used to create or output the image.

L*a*b color consists of a luminance or lightness component (L) and two chromatic components: the a component (from green to red) and the b component (from blue to yellow).

Custom Color Systems
These allow you to choose a color from a sample and print shops which support that system will be able to exactly match the color.

• PANTONE process guides let you choose from over 3,000 CMYK combinations
• PANTONE MATCHING SYSTEM includes 1,114 solid colors.
• TRUMATCH - Provides predictable CMYK color matching with more than 2000 colors.
• FOCOLTONE - Consists of 763 CMYK colors
• TOYO Color Finder 1050 - Consists of more than 1000 colors based on the most common printing inks used in Japan.
• The ANPA-COLOR ROP Newspaper Color Ink Book contains samples of the ANPA colors.
• HKS - Used for printing projects in Europe. Each color has a specified CMYK equivalent.

Complimentary Colors When discussing light, (additive theory) complimentary colors are any two colors that combine to create white. e.g green + magenta, red + cyan, blue + yellow. When discussing pigments (paint, ink, etc.), (subtractive theory) complimentary colors are any two colors that combine to create black.

Brightness

Besides differing in color (frequency), light can also differ in luminosity, or brightness. A table lamp emits less light than a halogen lamp, but even a halogen source cannot be compared with bright sunlight, as far as luminosity is concerned. Luminosity depends on the amount of available light. It can be measured and recorded in a numeric value. In the past, it was expressed in Hefner Candlepower, but nowadays Lux is used to express the amount of luminosity.

Absolute brightness is not very meaningful, because human eyes don't detect brightness linearly with color. Basically, we see Green as brighter than Blue. So, the term Luminance was invented, which is brightness adjusted to indicate appropriately what we really see.

Luminance is Gray tone values computed from RGB via the formula:

RGB Luminance value = 0.3 R + 0.59 G + 0.11 B

There are other variations of this formula also used, with slightly different numbers.

For example, a RGB color of (100, 150, 200) would compute its luminance as

(100 x 0.3) + (150 x 0.59) + (200 x 0.11) = 140
See: www.scantips.com/lumin.html

Brightness Values:

Candle light at 20 cm	10-15 Lux
Street light	10-20 Lux
Normal living room lighting	100 Lux
Office fluorescent light	300-500 Lux
Halogen lamp	750 Lux
Sunlight, 1 hour before sunset	1000 Lux
Daylight, cloudy sky	5000 Lux
Daylight, clear sky	10,000 Lux
Bright sunlight	> 20,000 Lux

Color Temperature:
Color temperature relates to the fact that when an object is heated, it will emit a color that is directly related to the temperature of that object. The higher the color temperature, the more 'blue' the light, and the lower the color temperature the more 'red' the light. Color temperature of light can be measured in degrees Kelvin (K).

Color temperature describes the color quality of a light source in terms of the amounts of red light and blue light. Color temperature is based on what is called a Planckian radiator, or simply a black body. As the temperature of the metal of the black body is raised, it goes from a dull black through red and orange to blue and finally to white heat.

Chromaticity consists of two independent parameters, often specified as hue (h) and colorfulness (s), where the latter is alternatively called saturation, chroma, intensity.

When judging the relative luminance (brightness) of different colors in well-lit situations, humans tend to perceive light within the green parts of the spectrum as brighter than red or blue light of equal power. The luminosity function that describes the perceived brightnesses of different wavelengths is thus roughly analogous to the spectral sensitivity of M cones.

If one chooses any two points of color on the chromaticity diagram, then all the colors that lie in a straight line between the two points can be formed by mixing these two colors.

Y = brightness or luminance

See Colour Temperature

LRV - Light Reflective Value:
Used for paints. The percent of light reflected from a surface (1-100) See LRV

When one is working in the world of physics on an atomic scale, energies are often expressed in eV (electron volts) rather than in joules.
For purposes of comparison, 1 eV = 1.6022 x 10 ^-19 J

1. Are there only seven colors? - Sir Isaac Newton in his early physics experiments decided the colors of the rainbow were Red, Orange, Yellow, Green, Blue, Indigo and Violet. Many of us remember them as ROY G BIV. Are there only seven colors? Newton believed in numerology and thought special numbers governed all natural phenomena. Seven is a very special number. It took God seven days for the creation, there are seven days in the week, seven openings in our head, seven seas, seven continents and many other important sevens.

• The original IBM color monitor CGA had only 4 colors.
• The EGA monitor introduced in 1984 had 16 colors.
• The VGA monitor introduced in 1987 had 256 colors.
• Most of todays monitors support 16.8 million colors.
• There are 216 colors described as Web-Safe because they work with most web browsers and VGA and above displays.
• PANTONE process guides have over 3,000 CMYK combinations listed by number - By picking a PANTONE color from a PANTONE reference book, you can be certain that the color which you have chosen matches the color which will be printed.

Ultraviolet:

 UVA (320-400 nm) - rays penetrate deeper into the skin
          and are implicated in premature ageing and wrinkling,
          as well as skin allergies and rashes. They are present
           all-day and all-year round.
 UVB (280-320 nm) - rays are responsible for sunburn.
         They are at their most intense around mid-day, during mid-summer.
 UVC - (10-280 nm)