Originally Posted by
Dave Humphries
IF
you have a scene that contains no strong colours but does contain a decent amount of white (that you haven't blown),
AND
you are viewing an overlaid RGB histogram
THEN
when when the WB is 'correct', the peaks will coincide
OR
when the WB is 'wrong', you will see the colour peaks separate
That said, this isn't a method I would use to even check an average photo's WB on, because it is too easy to get misled by real colours in the image.
So yes, while it is visible under some circumstances, I think it's the wrong tool for the job most of the time.