If you have a problem with the screen brightness then it's a problem with the hardware. Your screenshot looks absolutely fine cause it has nothing to do with the software.
I'd suggest you take it to Nokia for either repairs or a complete replacement.
Hmm.. Hard to tell from that pic. Although I had a similar but different issue on my 920. However, mine was very noticeable. I'm not 100% certain but I feel mine started after I dropped it from knee height screen first. No damage was visible but shortly after I noticed a yellow tinge forming on the bottom of the screen. One day it was visible on the top of the screen too. Then it slowly spread across the whole screen like a plague. My bright whites permanently became an off-white tan color. It was disgusting. Thankfully I was only a month or 2 away from my upgrade to the 1520 so I didn't care as much if it were a new phone.
Just watch to see if it spreads. If so I'd exchange it.