Interesting, thanks. If I understand how it works here's what I'd have to do to support changing the MC's hair color.
For every image the MC is in I'd have to render 4 images of only the character posed the same way but with black, brown, blonde, and red hair. Then I would have to add a variable for hair color and when each image is displayed show the scene and the appropriate character image on top of it.
What about sex scenes? Arms and legs intertwine so I assume I'd have to include the sex partner in the 4 renders. What if the character is partially behind something? I assume I'd have to include that thing in the renders as well. The alternative in both these cases is to change the pose and position of the MC to avoid the situation but that would mean sex scenes without touching each other. Maybe all of this is why many games do 1st person characters with little floating hands, tongue, and penis?
If I understand everything correctly it would be a ton of work to retrofit into the game. There are somewhere around 1800 images in the game and I'd guess that somewhere around half include the MC. Even if I was starting from scratch and only considered the MC's hair color I would never add that much work. More importantly, who would be satisfied with only hair color? Skin color, clothing, etc. Then someone would want to do this with the teacher and store owner too! With all the permutations and combinations there is no way I'd ever finish the game adding work like this.
The world is full of compromises! Cheers!