While on our recent visit to Shapeways in Queen's, NYC, we noticed vats of color dyes. Why color dyes? It's all in the finishing.
Some people expect "finished" objects to emerge from their 3D printer, but in fact they often require "finishing", which might be simply removing excess plastic material created during printing or perhaps sanding.
But Shapeways explains another important finishing process in a recent blog post: dyeing. Prints can be painted, obviously, but dyeing enables more interesting color applications, as you can see in the image above of Shapeway's maker Rachel Goth's work.
Dyeing can be a very inexpensive and effective method for anyone using a monocolor 3D printer. Just make sure you print in white; that will make dyeing a bit easier.