RWBY theory:
Aura is most commonly singularly-colored and specific to the individual. However, there's a possibility that Aura can be dual-colored in the very least. Such an occurrence would have to be rare, as the majority of Humans and Faunus who can project their Aura have all been shown to only have Auras of the singular variety. That said, there may be individuals - both Human and Faunus- that can have multi-colored Auras. One has to wonder what the criteria for such a variation would be.
I think that if an individual had a strong enough affinity to multiple colors, to the extent where it could be a part of their identity, then their Aura would reflect those colors.
We know for a fact that Aura is the physical manifestation of one's soul, even if they can't project it. Likewise, one's soul is tied with their personality and their ego; what makes that individual themselves, essentially.
So, where am I going with this? Well, in my opinion we've seen one person who may fit such an outline...
Neo Politan.