I always use Rainbow Dash or Dash when I'm talking, and I'll also use RD when typing. I suppose Rainbow makes sense, but I agree with Mecha. She is rainbow coloured, but she loves to race and we really identify her with her speed. Even thinking about the sonic rainboom gives me an image of incredible speed. As for the other two, I think saying Pinkie is a lot less ambiguous than Pie, and it also suits her better (she's always happy and cheerful, and pink is a very vibrant colour). Twilight makes more sense to me than Sparkle because she looks more like a twilight-ish colour; if you think of her as being Celestia's student, and her being between Celestia and Luna on the spectrum... I dunno, maybe it's only me.