Awesome links up in here! I must say that Allysin Chaynes sure has a hardcore pussy: DAYUMN!
Anyways, to answer your question cc100: They need bleaching because it makes that region of the butt more, in terms of "aestheticism", in tone with the skin. Just to keep the whole body's skin clean and in-tone.
They go brownish over time. Other times are because of too much anal, I guess. And others, well, pretty obvious. Even twats get a darker color as well, so, I presume that that "brownish" hue is for too much "action", if you catch the drift.
Honestly, I don’t mind it, but it does really look nice to see that region cleaned.