Background Color after Masking

e.g., if you mask an object in the color page and send it via "add alpha output" to the media page, die "background" used to be black. In one of the YT videos there was described a way how to change this to a color. I unfortunately played around with this and set color to red. Now, if I mask something and come back to the edit tab, I get red all over the place - and cant get rid of it.
Meaning, I dont find the switch in the preferences to go back to black
PLEASE HELP

It is not in preferences, it's in the timeline menu that has the track height adjustment sliders.

This is where to look.

Thank you, Peter and Charles! You saved my day!

Yeah, that's a hard one to find.