3d printing has come huge strides in ability to construct detailed objects. Unfortunately, color is still a considerable limitation. Here, some people at the Reprap blog are having fun coming up with an extruder head that actually mixes two colors as it deposits them. Don’t confuse this with the dual head that Makerbot is touting that allows you to switch colors on the fly, this is a single head that actually has a cavity where the material is melted, then stirred to create a combination of the two. It is an interesting method of overcoming a limited supply of colors.
Having this extra stirring chamber means that there would be a small amount of material wasted any time that you wanted to make a change to the color, as it would have to be purged. There are some interesting thoughts in their comments on how to use this extra material most efficiently.