User blog comment:629638/Black and white/@comment-4010415-20140306105704

We barely know anything about dust. We've only been able to guess what each color does. The only really obvious thing we've seen is in episode 2, when Ruby sneezed on a combination of red, yellow, and blue dust (you can barely see the traces of yellow and blue dust with the red).

From what I can tell from that scene red is fire, blue is ice, and yellow is lightning.