sorta like blendy pens?
if so, (does anyone remember the fill can? it could that model) each color that is added will make the variable color(like going through select color - more colors menu, in ms paintin it, you can use the rbg bars to select amount of each color, createing a variable.)the script for this would have to have a "add color values" thing were it combines the levels of rbg, makeing variable color.
if not, i have know idea what you would do.