Sorry, it's been confirmed that the issue is hardware related. The downgrading probably didn't cause it, but made you aware of it.
The sensor actually gets damaged over time. Some people traded in their ones, made sure there was no tint, but still ended up getting it anyway.
Moral of the story: HTC sucks for selling faulty cameras.
PS: I sent my phone in for warranty a couple months ago, after having the purple tint for a year, that seems to have permanently fixed it.