Final Cut Pro X plugin maker osmFCPX has produced a helpful tutorial that shows you How to use Adjustment Layers in FCPX without screwing up your Background 

Also check out Ryan Welborn of osmFCPX on a recent episode of the excellent FCPX Grill podcast where he discusses how he got into making plugins for fcpx. LISTEN HERE 

You can purchase osmFCPX plugins on the FxFactory store