Maybe your media player modifies the gamma settings.
To make sure if that has something to do, you can try this:
- open a terminal and run "xgamma" on it, note down the output of remember it (usually 1,1,1 for red,green,blue)
- run your video player
- run xgamma again in the terminal, and see if the values changed
If the values changed then your media player is changing the gamma factor for some odd reason. Look in the preferences dialogs and check if there's something about gamma or brightness and disable it if possible.