Icing for John Lucas
Not much working time for mixing in the color (e.g. food coloring - hint, hint), but how about spackling compound? I've used it un-colored to simulate cream cheese on a turned bagel. I definitely would NOT use it in a cake decorator, though, unless it was expendable.
IIRC, you work at a university. If there's a chemistry department, they might be able to suggest a retarder for the spackling compound, so it could be used in a cake decorator.
FWIW, Will Vinton (Claymation, California Raisins, etc.) used to mix his own colored modelling clay from raw white plasticine, to get custom colors.