As far as I understand it:
Levels() affects all three Y, U and V and clips the levels.
YLevels() changes Y only and leaves U and V untouched. It does not clip.
One may use 'chroma copyback' with Levels() in order to leave U and V untouched, like
Code:
mergechroma(Levels(24,1.0,235,16,235,coring=false),last)