Basics - GIMP 2.6 for Photographers: Image Editing with Open Source Software - page 9

If you wish to specify a color for a printer or want to choose a unique
background color for your web page, the following information should be
considered.
Specific colors correlate to unique numerical values. In the RGB color
model, each of the primary colors (red, green, and blue) has a decimal color
value ranging from 0 to 255, with black as 0 and white as 255. Hence, there
are 256 color values for each of the base colors red, green, and blue. The
total number of potentially resulting colors is calculated by the following
multiplication:

256 • 256 • 256 = 16,777,216 colors

To specify the number of colors of an image or color model, I use the term
color depth, which is specified in bits. The RGB color model has a color
depth of 24 bits (24-bits = 2 to the power of 24 colors = 16.78 million
colors).
These values apply to color as well as black-and-white images. In the world
of digital image editing, black-and-white photographs are called grayscale
images . In addition to the black and white “colors”, grayscale images contain
all possible shades of neutral gray.
Since the color values of the three primary colors must be identical in
order to produce purely gray levels, the number of gray “color” values amounts
to 256.

Grayscale images have a color depth of 8 bits .

In the RGB color model, the colors are normally defined in decimal
numbers . As mentioned earlier, each single color can have a value between
0 and 255. You can use the eyedropper icon located in the Toolbox of GIMP to
open the Color Picker to measure a color. The Color Picker will show you the
number corresponding to a color's exact value so you can easily transmit the
information to your colleagues or work partners.

Color
Black

0 0 0

Red 255 0 0

Green 0 255 0

Blue 0 0 255

Yellow 255 255 0

Cyan 0 255 255

Magenta 255 0 255

Medium gray 128 128 128

White 255 255 255

The primary and secondary (mixed) colors in decimal notation