The /usr/openwin/lib/rgb.txt database translates color names into red, green, blue values. You can refer to the rgb.txt file for color names to use for your site's labels. You can also use hexadecimal color values.
Briefly, here are a few high-level points about color values:
Color values specify the amount of red, green, and blue (RGB) that compose the color.
RGB values can be specified with three hexadecimal numbers from 0 to FF. Each hexadecimal number indicates the amount of red, green, or blue in the color.
For example, pure red is #FF0000, pure green is #00FF00, pure blue is #0000FF, pure white is #FFFFFF, and pure black is #000000. For more information, see the X11(5) man page in the /usr/openwin/share/man directory.
The number of colors that are available on the screen depends on several factors:
Amount of memory available for specifying colors
Number of color planes
How many other clients are using color cells
Whether private color maps are being used by other applications