You probably have heard of the 3D Printer manufacturer, Builder. They are the makes of the Big Builder, a 3D printer with an enormous build height (Z-axis). They just announced today, the introduction of a new tool that will work with their Dual-feed extruder on both of their 3D printers. It allows for users to mix colors using a web app prior to printing, then generate g-code and print. The results? The ability to mix colors (see image below).

Read more about this at http://3dprint.com/5235/builder-color-mixing/