Agree with tonschk regarding the fog, if it rolls in then it'll make everything look more dull with less darker colors.
Also your monitors setting can play a big part with tuning rather than just relying on your video card settings alone. My wife works with photography and bought one of those devices that you place on your monitor to calibrate it and my first reaction was... meh I don't need that, I already have it close.
Well she insisted I use it and after an hour later of doing so.... man, I was way off with the settings to go along with the room light. About the only thing I had close was the contrast. Well I kept adjusting the settings through each step it took me through and got it spot on to what it was suggesting.
I could see a difference right off and the first time I started a game I noticed the colors looked much richer than before. Her photos also come out the way she intended for them to look after working on them and sending them to be developed.
