Wait so their color changes after masturbation????
Don't know if that's a serious question, but no, they don't. It's a myth/visual metaphor used to shame girls and women into not doing anything deemed abnormally sexual, like masturbating, or enjoying sex at all, lest their bodies become tainted and inherently dirty. Same genre of sex fearmongering as "you'll get loose if you do it too much", or "you're like a chewed-up piece of gum", or "you lose a piece of your soul every time you have sex with someone", except when they say that slutty girls have darker vaginas, they're also saying that girls who just naturally have darker vaginas are slutty, so it's double messed up.
Anyway, I feel like it's super rare and kind of interesting that a loli sex manga has a cover that's 100% about the busty adult teacher, and the loli doesn't appear? You'd usually think that if someone's okay with the lolicon, then the lolicon would be the draw, but it seems like they banked more on the demure teacher's cleavage as a selling point. Thought that was curious, anyway.