You must've accidentally set the resolution to 1680x1050x16. That means you've put it on 16-bit color depth, which means that the computer now has 1/256 the amount of data it did before to describe what color each pixel should be. So, the computer has less colors to choose from and some turn out kinda odd, like you said.
Make sure the resolution is 1680x1050x32, that will give you 32-bit color depth, give the computer more colors to choose from, and return the look to normal.