Humm... very good question, I looked at the code and it looks like that part was never ported to use the skin system, so it all uses hardcoded values...
the colorbar is divided into three parts, first part is from pixels 0 to 10 and goes on the left, the second part is the 11th pixel being repeated, then the last part on the right are the pixels 12 to the end from the colorbar...
You think it should be skinnable? then which options do you suggest ?
And could you provide a patch?

(look at skins.tcl, function getColorbar)