Hi kenchan
You can mute the video on the track by clicking on the mute button in the track name. (see screenshot).
You can also mute the global video by clicking on the mute global video button (see screenshot)
You can also control the brightness intensity of the global video mix by adjusting the video intensity control bar (See screenshot)
These options should be what you are looking for.
If you have any more questions, please do not hesitate to contact me.
Best regards
Cormac
Click thumbnail to see full-size image