I've never really thought about whether the game looks the same using its Windows vs Linux renderer. I installed HL2 to check out the anniversary update and decided to try the Windows renderer on Proton. Would you look at this difference... Linux is the top one, Windows is the bottom:
Is this is a thing? Have you tried other games and seen the Windows version looking better?
It just looks brighter to the point it could be different gamma settings.
There is a bigger brightness range in the top screenshot the bottom looks washed out.
Could be. The gamma setting is the same across both instances - 1.6. But it's possible that the two instances render that differently.