1. In Advanced performance options of Media Player 10 (Tools => Options => Performance => Advanced) there is an option to use either High Quality Mode or Overlays. I've encountered issues with complete video desaturation when using High Quality Mode. This has to do with MP10's incompatibility with nVidia drivers (It is a MP10 problem because this issue did not exist with MP9). Use Overlays instead. It is NOT really a problem at all. In Desktop Properties => Advanced you can adjust how your hardware handles overlays (saturation/gamma/sharpness/digital vibrance) for each mode (such as Desktop, Video, etc.); I am pretty sure that this will resolve your prblem for MP10. What is happening with Quicktime I cannot rightly say. But it is possible that it has similar options. Just try using overlays.
2. Download and install K-Lite Codec Pack. (It may associate all video formats with Media Player Classic. You will have to change associations to Media Player 10 and Quicktime if you want to use them); If this does not help, then you can rule out codecs. Still, it is good to have the K-Lite Codec Pack because it will enable your system to play just about any video format in existence.