What are you using to play the Video clips......Windows Media Player 10 has an array of sliders, on of which is one for brightness.......to see the sliders ......click View/Enhancements/Video settings and try that. If you don't see the correct slider, there are left or right arrows where it says video settings.............. Any other player will have similar adjusters ...............commoner.
BTW....you must have the video clip actually playing whilst using the sliders, once it stops they don't do anything..