IPhone 4S screen replacement gone wrong?

Hey guys got an iPhone 4S screen replacement about 2 weeks ago and only just noticed when putting it against my girlfriends iPhone 4S that mine is darker I put both phones on full brightness and together mine looks very slightly blue and also darker but my screen is new? What have they done to my phone and what are my rights I'm in the uk England if it helps. Please tell me the legal way to deal with this before I go up there and get angry.

I'm from the uk too! Make sure that when you go out and get replacements for anything, take a look at the store/shop and see whether they have a license on show (so you know there not scamming you) and is it a propper well known store that has good ratings. I would check the price and compare them to other shops. If they're not well known, they haven't got a licence and they charge a lot then I would complain to the manager or get a refund.
Hope that helps.