Phoenix FD 3.04 Quick Tutorial - Mixing Smoke Colors
Here is how easy you can mix colored smoke in the same simulator or emit varying colors from the same source :)
The important steps are:
enable Grid RGB in the Output rollout of the Simulator
enable RGB in the Source
set Based on for the Smoke Color to RGB form the Simulator's Volumetric Options
You can also emit RGB from a texture in the Source.