While http://purl.org/tcl/home/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.
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Okay, here's the standard (and rather reliably portable) 6x6x6 color cube.  This page has been
around forever:
   
   http://the-light.com/colclick.html
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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.  Programmatic access to the English-to-RGB-code map is available as
  winfo rgb . $color




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[chooseColor] answers some questions on this subject.
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The [W3C] maintains a list [http://www.w3.org/MarkUp/Guide/Style#colors]
of "Web-safe" colors.
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xcolors is an interesting command under X11.

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International Color Consortium maintains a page [http://www.color.org] which has information on ICC color profiles for computer color matching.

Mildly related to this is "[Selecting visually different RGB colors]".
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'''Percent to color''': The following routine produces a color from an integer between 0 and 100, where 0 is red, 50 is yellow, and 100 is green (useful e.g. for painting progress bars):
 proc percent2rgb {n} {
    # map 0..100 to a red-yellow-green sequence
    set n     [expr {$n < 0? 0: $n > 100? 100: $n}]
    set red   [expr {$n > 75? 60 - ($n * 15 / 25) : 15}]
    set green [expr {$n < 50? $n * 15 / 50 : 15}]
    format    "#%01x%01x0" $red $green
 } ;# RS

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