The colour change is makeup mode it's in Mizuki's new Transform! dialog option along with the neko mode, it's just a player two style colour swap but it does also work with neko mode and the costumes. The things activated by the H key are the cheat keys that were removed from the normal version, there's a txt file inside the games folder listing them if you need a reminder of what they are.
I still don't know why tan lines were specifically mention in the change log she already had them in the earlier versions, just because the new colour swap also has them too I guess (the list is very itemized).