[plt-scheme] slideshow, draw letters in a word in different colors

Is there a way to do kerning in slideshow?  I have a string, say, "ABCD".  I want each letter to be in a different color, and I'm not using a monospace font.  It's easy to do if I wanted spaces in between the letters A B C D, but I want ABCD.  I've tried using cbl-superimpose and prepending some arbitrary number of spaces ("A" " B" " C" "   D"), but that's pure guesswork and I'd rather do this precisely and predictably.  I think I can make a pict out of each letter and get its pict-width, but then I still have to figure out how to do the kerning.

Geoff