PodMaster gives a good answer. As for the named colors they seem to be the same as the valid names for HTML and X11 colors (are these identical? does anyone know?) Anyway, for a quick reference I usually take a look at DevGuru's HTML color chart: http://www.devguru.com/Technologies/html/quickref/color_chart.html. I have yet to find a named color on that chart that wasn't accepted by Tk (though, I have not tried very hard - YMMV).

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