Just did some thinking, and I put some dots together about Pupil Colors.
For the most part I'll only be talking about Canon skins.
I think Red means multiple things, one is another form of Yellow (Served their respective community) as in Badgy (Scientist), Daisy (Military), Beary (Doctor) and so on...
Another is they took one of the potions, like Doggy, Penny, Sheepy (I think she did?) and Badgy (Came in contact with it on accident).
To back this up, the normal Doggy skin has one yellow pupil and one red, while the officer Doggy skin (who wasn't forced the potion) has two yellow pupils.
I think Purple/Magenta means they were afraid or worried near their infection.
Such as Grandma (Worried about Georgie in her note), Mousy (Her legs were cut off).
Blue, I'm not entirely sure, but I think it could mean they were betrayed or infected on purpose.
Like Foxy (Locked in and infected by TSP) and Poley (Infected by TSP)
Obviously white and red means they were in TSP.
Anyways, that's it. CYA!