It is possible to construct (almost) all visible colors by combining the three primary colors red, green and blue, because the human eye has only three different color receptors, each of them sensible to one of the three colors. Different combinations in the stimulation of the receptors enable the human eye to distinguish approximately 350000 colors. A RGB color image is a multi-spectral image with one band for each color red, green and blue, thus producing a weighted combination of the three primary colors for each pixel.
A full 24-bit color image contains one 8-bit value for each color, thus being able to display different colors.
However, it is computationally expensive and often not necessary to use the full 24-bit image to store the color for each pixel. Therefore, the color for each pixel is often encoded in a single byte, resulting in an 8-bit color image. The process of reducing the color representation from 24-bits to 8-bits, known as color quantization, restricts the number of possible colors to 256. However, there is normally no visible difference between a 24-color image and the same image displayed with 8 bits. An 8-bit color images are based on colormaps, which are look-up tables taking the 8-bit pixel value as index and providing an output value for each color.
©2003 R. Fisher, S. Perkins,
A. Walker and E. Wolfart.