I mean because her cutie mark is black.like.the color black.but I guess sometimes magic matches hair color.(vinyls Canon magic color being blue,and no other part of her is blue.and my theory about magic color.I'm not gonna explain the whole thing,but I think twilights magic matches her hair rather than her cutie mark.because she was seen as a filly (also before she got her cutie mark)using magic.in THE CUTIE MARK CHRONICLES and A CANTERLOT WEDDING and the only other part of her that is magenta is part of her hair.and also perhaps it was just a coincidence that her cutie mark was also that color.I'll post my theory on my profile.(that's gonna take a long time.)