Hey, I know this post is like 2 years old but I am having the exact same problem. My TV tuner is playing TV in black and white and all my avi's, mpegs etc are playing in black and white! I dunno what the hell is going on :S
You need to go into Nvidia control panel (assuming you have a nvidia graphics card) and you need to look under 'Television and Video settings'. In there you should see an option to adjust "Saturation". Turn this up about half way (or just equal to all the other controls) and all the colour in your videos will be back