Board Thread:Speculation House/@comment-124.170.71.149-20131218072814/@comment-4010415-20151121215440

Yeah, there are four basic types, and they're fire, water, electricity, and wind. You get earth Dust by combining two types of Dust to make a new color, and then you use that new color.

Though, we don't know what types you have to combine, or if some types require that you combine three types.

Apparently, though, they have planned out what combinations result in what types. It's written down in their RWBY bible, according to Miles.