Colors in Computer Graphics

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    URL:  http://alfonse.bitbucket.org/oldtut/Illumination/Tutorial%2009.html

In the real world, our eyes see by detecting light that hits them. The structure of our iris and lenses use a number of photorecepters (light-sensitive cells) to resolve a pair of images. The light we see can have one of two sources. A light emitting object like the sun or a lamp can emit light that is directly captured by our eyes. Or a surface can reflect light from another source that is captured by our eyes. Light emitting objects are called light sources.

The interaction between a light and a surface is the most important part of a lighting model. It is also the most difficult to get right. The way light interacts with atoms on a surface alone involves complicated quantum mechanical principles that are difficult to understand. And even that does not get into the fact that surfaces are not perfectly smooth or perfectly opaque.

This is made more complicated by the fact that light itself is not one thing. There is no such thing as “white light.” Virtually all light is made up of a number of different wavelengths. Each wavelength (in the visible spectrum) represents a color. White light is made of many wavelengths (colors) of light. Colored light simply has fewer wavelengths in it than pure white light.

Surfaces interact with light of different wavelengths in different ways. As a simplification of this complex interaction, we will assume that a surface can do one of two things: absorb that wavelength of light or reflect it. 

A surface looks blue under white light because the surface absorbs all non-blue parts of the light and only reflects the blue parts. If one were to shine a red light on the surface, the surface would appear very dark, as the surface absorbs non-blue light, and the red light does not have much blue light in it.

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      URL:  http://www.learnopengl.com/#!Lighting/Colors

The colors we see in real life are not the colors the objects actually have, but are the colors reflected from the object; the colors that are not absorbed (rejected) by the objects are the colors we perceive of them. For example, the light of the sun is perceived as a white light that is the combined sum of many different colors (as you can see in the image). So if we would shine the white light on a blue toy, it absorbs all the white color's sub-colors except the blue color. Since the toy does not absorb the blue value, it is reflected and this reflected light enters our eye, making it look like the toy has a blue color. The following image shows this for a coral colored toy where it reflects several colors with varying intensity:

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You can see that the white sunlight is actually a collection of all the visible colors and the object absorbs a large portion of those colors. It only reflects those colors that represent the object's color and the combination of those is what we perceive (in this case a coral color). 

These rules of color reflection apply directly in graphics-land. When we define a light source in OpenGL we want to give this light source a color. In the previous paragraph we had a white color so we'll give the light source a white color as well. If we would then multiply the light source's color with an object's color value, the resulting color is the reflected color of the object (and thus its perceived color).