Silly observation really. As far as I can tell, Kajishima probably just liked the aesthetics of it.
But wouldn't it have been as practical and easier to understand if Fuku and her cabbit clone posse were given different colors?
I guess you could explain that, on the island, they didn't want them to be recognizable- making it difficult to recognize how many were around. But once you get to the OVA, it's just kinda annoying.
Fuku could maybe have a stripe? or maybe a red hue to match her gem?
Miki would be brown/ brunette to match her natural hair.
Kirche would be white
In an anime world, nothing wrong with D being green... or maybe black? or a kind of tan with a black ear that always covers one eye?
could have been so much fun.
They are going to a tropical island. the white hair reflects more light to keep them cooler.
It would be interesting if Kajishima-sensei had an actual reason to make the cabits white. Aren't they clones of Fuku? So one would think they'd be Fuku's coloring. But then maybe the white indicates they are possessed by one of the Dual! girls.
It is said that they changed their color so the pirates on the island wouldn't easily make the connection to Fuku. But no reason was made to why they all had to be white. And once the proxy war was finished, there shouldn't be a reason to still keep them all white.
It hit me in the scene where Seto, Misaki, and Yuzaki are questioning Seina's girls on their sex life and Misaki is holding, I assume, Miki (because of the disdainful glare) but because she's white, you can't be sure.
In case anyone missed this character's cameo:
https://tenchi.fandom.com/wiki/Kirche