Hmm... I can't see anything in the settings that might cause that problem...
Have you checked Media Player to see what brightness/contrast it is set to?
This might help me figure out whats wrong, because I have a feeling that, you know your video card settings can hold settings on the brightness and contrast of videos? Well Media Player might just be overtaking these settings for its own, and so culd that program.
Go into your video card settings:
right click desktop, Properties, Settings tab, then to advanced at the bottom.
If you had an ATI card for example it'd be under Colour settings of the card, but Nvidia i don't know.
There should be some settings somewhre though to change the brightness/contrast values of videos that might be affecting playback colours/contrasts.
Set these to higher than they are, or to standard middle values if they aren't already and try running some videos again
Hope this helps