The Color Definitions File
The rgb.txt file shipped by MIT jointly with the X Window System contains predefined colors in the form of correspondence between their specific RGB values and symbolic color names. The color values are present in the RRR GGG BBB format, where R, G and B represent single decimal digits and determine the intensity of the red, green and blue primaries that make up each color.
Some sample lines in the rgb.txt file may look like the following (in the "RRR GGG BBB color's alias" format):
160 32 240 purple
255 0 0 red
255 0 255 magenta