^They are always drawn pink with black bone in official zun stuff- I went and had a look- but overall i'd say she is most often drawn in doujins with them all black which is why I was a bit confused. Even some with her pink wings on their covers have her wings all black inside. I'm just gonna guess its just done to make it quicker and easier to draw them over and over when in black and white.
In coloured covers or fan art even though, half the time she is just given black wings- maybe influenced by all the doujins- or people just think black bat like wings look better?