While http://tcl.activestate.com/man/tcl8.4/TkCmd/colors.htm , included in the standard core distribution, completely answers most questions about the color values available for syntactic use in Tcl scripts, several other facts about color might interest programmers. This page aims to collect all this sort of information.
Okay, here's the standard (and rather reliably portable) 6x6x6 color cube. This page has been around forever:
http://the-light.com/colclick.html
The source file which realizes the description which appears in the hyperlink at the top of this page is xlib/xcolor.c. X11 (Unix) systems often have a showrgb command and/or .../lib/X11/rgb.txt which give the same (?) data.
The W3C maintains a list [L1 ] of "Web-safe" colors.