I had an interesting idea while taking a look at this thread:
"Experiment with Colour Printing"
What if, instead of applying color through some automatic coloring device near the nozzle, the color is applied manually beforehand precisely where it is needed? I have not explored the feasibility of this, but I am willing to bet that the visible part of a print does not amount to much in terms of filament length. 'All' this would require would be a program that could integrate a sliced 3D model and a textured version of the model, find the edges of the sliced model, match them with the colors in the textured model, and give the user a readout of where to color their spool of filament a certain color.
It would be interesting to try it out, but I have no idea how complicated the software would be to write. Does this sound like a good/useful idea?