Been bit by this a couple of times and although I’ve fixed it each time I forget what I did the next time I encounter it. :) So here’s a note for myself and anyone else …
If you Big Sur menu bar goes all white like this:
Fear not, simply hide it and unhide and it will return to the regular colours. For whatever reason that’s the only trick which seemed to work – doing things like relaunching Finder, locking the screen, or changing between dark & light modes don’t make a difference.
To hide and unhide the menu bar go to System Preferences > Dock & Menu bar > and the bottom you will find “Automatically hide and show the menu bar”.