I think the red helps her look and feel, I dunno, more intense somehow. I always felt it worked just right with the yellow to show how she is like as a person visually. It contrasts better - after all, her hair isn't supposed to be straight up fire. As for yellow, well, Applejack or Pinkie have no less of those, and all these examples have different shades of colour that don't feel the same at all and work just fine. But different strokes for different folks, I suppose.