MakeAnything did a cool project with coloring filament using sharpie markers, and I modified his file to work with my Prusa. Its a tight fit, you have to be careful especially with the back marker, that it doesn't get snagged on the frame. Mine is printed in a granite PLA filament, and in the pic it is printing a benchy in blue and then orange. Colors are mostly just mixing and staying flat so far. The filament that works best to color is anything branded as "natural" or undyed PLA.