Assuming for the moment that the hair colors had no greater significance (I do kinda like the relationship-stage theory, though), purple-Dora would have been much handier when Raven was around. Even with Raven's mole, they frequently looked too much alike. As did Dora & Marten during most of Dora's black phase. Sister, hell, try "twin"! And there might also be quibbles about how hard it is to distinguish Sven from Angus sometimes, but that's a different topic. At least the ladies can resort to hair dye to correct that, as we've seen.
I liked the black-tipped blonde on her as well. That brings to mind another goth/punk/etc. hair combo Dora could try -- black with red tips.