Video background removal

[Bálint5D93]

Hey y'all!
I have a video on top of a picture in my timeline. The video is of a guy falling over. And i need to remove the background of that video so that the picture underneath is more  visible. 
Have any advice or tutorial video you can help me with?
Thanks in advance! ❤️

[RobShultz]

If it was a solid color I would recommend the Ultra Key effect. But if it isn’t a solid color you might want to look into the rotobrush effect in After Effects.

[MarekMularczyk]

What's the background? Is it a green or blue colour (plain)?

Can you share a screenshot?