I’ve seen different headcanons about why Crosshair has silver hair, and I think most of them involve something with his advanced aging gene. I feel like Crosshair would be super sensitive about it, like to him it’s a sign that even his defects are defective, and he’s constantly worried that the aging issue will affect the rest of him and not just his hair. And I feel like maybe the others are aware of this, so one day they sneak a bunch of posters of celebrities with silver hair who people still find incredibly cool and attractive into their room and start dropping hints like “man that guy looks cool, I wish my hair would do that” and “dang I’ll be lucky if I even HAVE hair when I age” and Crosshair will never admit it, but it makes him feel like, way cool. And a lot, lot better about his hair
The Wookieepedia page on PROXY doesn't say what his specific origin is, but it does say he's a prototype created to help with training Vader's apprentice.
Headcanon time!
Vader made PROXY. Which, since Anakin also built Threepio, makes PROXY Threepio's little brother, from a certain point of view.
Headcanon that padawans and initiates try to sneak into the Senate building when the Senate isn't in session, so they can play bumper cars with the hover pods. The 501st hears about the tradition from Ahsoka, and of course they also get in on it!
(Extra points if this somehow ends up with Palpatine accidentally assassinated by a speeding hover pod.)