alright there's not much info about William, and my first headcanon is a bit more Supernatural so I don't know if it fits. Therefore, two for the price of one just to be safe:
1) William's hair is Completely Natural. He doesn't dye it. Half his hair is blue-green and the other half is red, and that's just how it's always been. People assume he dyes it but if you look really closely you can see his eyelashes have the same deal going on. He's been wierdly self-conscious about it for most of his life, after all, most people don't have two-colored hair OR blue-green hair, and yet he has both. It's a medical mystery, that's for sure. When he was younger, he'd just bleach his whole hair blonde and not worry about it, but as he grew up, it became more bothersome bleaching his hair than it was to worry about what others would think. Nowadays he's more ambivalent towards it.
2) If the previous headcanon is too fantastical, an alternative: He's a sorta ambivert type of person. Everyone assumes he's extremely introverted; after all, he doesn't talk to other people much and spends a lot of time focused on his own work. However, he actually really enjoys social interaction and finds it energizing. It's just the problem is he's enough of a workaholic that, even though he enjoys spending time with others, he doesn't appreciate it enough. He forgets how good it can feel to spend time with others as he gets lost in his own day to day routine. If you invite him out, it'll give him an excuse to have a good time and he'll be really happy. He's not really consciously aware of this problem he has.
(alternatively. both headcanons. they're both good.)
EDIT: 3rd mini-headcanon because it just kinda Occured to me. Okay, so like. he's got a lot of likes and dislikes related to food, yeah? Okay here goes. He is totally an epicure. Like, he's really picky about food quality and knows what makes food gourmet or not. He's a really, really good cook too and he'd honestly be happier as a chef than his current job but he sells himself waay short when it comes to food. Like, he knows he can cook well but he doesn't know just how good his cooking is; he takes it for granted and assumes lots of people can cook way better than him.
Ooh those are nice! Yea, first one's already (semi?) canon; I've described her as having a silly putty like consistency. Second one tho? It's a great idea and I'm gonna make it partially canon with the caveat that she's gotten better with practice. in part cuz her own preferred human form is one she's made up herself, after all.