Basics - GIMP 2.6 for Photographers: Image Editing with Open Source Software - page 8

1.3.3 Screen Colors - Color Models and
GEGL, the New Graphics Library

GIMP's version 2.6 employs three color models: RGB (red, green, blue),
grayscale, and indexed.
GIMP uses the RGB colors, or colors of light, as its default. Together,
these colors form what is known as the additive color model . It uses the
three primary colors—red, green, and blue—to create a color spectrum
containing approximately 16.78 million colors. This is called true color because
it represents the maximum number of colors that a computer monitor or
television screen can display.
Mixing two primary colors in RGB mode will result in the creation of a
secondary color, such as yellow, cyan, and magenta. No color (or the absence
of light) creates black, while the sum of all colors results in white.

Figure 1.4
The RGB color model

Figure 1.5
The color spectrum of the RGB color model.
About 16.78 colors are shown, with black at
one extreme and white at the other.

Figure 1.6
Approximated representation of the set
of colors in the RGB color model

Figure 1.7
There are 256 gray levels in the RGB color
model.