[APN] The Colors package implements commands for manipulating and converting color values (Tk names, RGB, HSL). Not sure of the original source but a copy is available from [https://sourceforge.net/p/irrational-numbers/code/HEAD/tree/pkgs/Colors/trunk/colors.tcl].

Also possibly useful are utility routines from Jeff Hobbs for contrast, shading etc. Now available from [https://sourceforge.net/p/windowstoolset/code/ci/default/tree/apps/wits/color.tcl].