In the case of Brave, I'm sure he just exaggerates a lot, because hyper fuckhuge titties and everything is big, and warped to the point of being comical, BUT in general (the idea of nipples and pussy lips themselves getting darkened) is something akin more to the idea of a "visible" or a means to denote corruption, or in this case a form of progression, to said level of corruption (and moral degeneration) in the girl and/or woman.
There are artists on Patreon and Fanbox that I've seen do this change pretty well, some like Takeda Hiromitsu, Dramus, come to mind, but it's also a divisive one, given your response and I could imagine other people, seeing as offputting, visibly ugly and other things.
It's also another factor, in NTR stories and just hentai as a whole, to do a contrast, of how the woman started vs. how she is/ends up, and in the case of a storyline of corrupting and "dirtying" her into becoming a slut/whore for everyone except to the man/beloved person she is originally in love with, some people also add this into the aspect of that woman or character becoming more and more depraved, having times when either the one fucking/taking the girl, starts to "imprint" in a sense, some of the style and mannerisms he has onto her, and that's also why you can see a change in how visually said girl gets messier, naughtier, I guess ugly in this particular case, to the point she becomes unrecognizable of once who she was, among other things. But also that process of seeing the steps that woman takes when she gets eroded, to what she becomes, is also what many consider so appealing in stories like these.
The problem with Brave, and this game in particular, is that like 99% of the girls he draws in his sets and images, the moment they get fucked, they already become a perma turbo slut (or are already one to begin with), skipping all the progression, and natural changes one would see if the story, and the girls in this game, got a gradual, consistent development in which they "fall" and end up breaking, but with this game is like going from 0 to a 1000 in just the next scene, and it can be very jarring, especially when stealing/getting the POV of the one being cucked and by the next scene she's already being fucked by half the entire male population.
At that point, as some already pointed before me, it's just a fetch quest of searching the scenes and that's it.