I was working with Aqua Regia in the lab today and was impressed by how quickly the solution (HCl and HNO3) changed colors from a colorless liquid to orange to a deep red. I didn't time it, but I would estimate that the change from clear to orange took under a minute while the color change from orange to red took about an hour.
Why does this color change occur? Why does the first stage (clear to orange) occur so quickly without mixing? I'm sure this information's out somewhere, but I couldn't find an answer with searching.