I suppose that this only happens with NVIDIA cards. Yesterday I just upgraded to Karmic Koala and today I discovered that even though everything seemed normal, video playback appears in false color and in a shade of blue, like this:
I tried to play with the NVIDIA X-Server Settings and I discovered that the Hue parameter under X Server XVideo Settings had a value close to -1000, instead of 0 which is the normal:
If you click the Reset Hardware Settings button all values return to the default (0) and your video will appear in true color again:
To open the NVIDIA X-Server Settings go to: System → Administration → NVIDIA X-Server Settings.
Certainly this is rather a work-around than a solution, but it allows you to watch video until a bug fix is released.