As a piano technician, I agree 100% with the advice not to use contact
cement on ivories, although I was introduced to it early in my career
by another technician who said I would be throwing away all my ivory
clamps.  With any glue, it is VERY difficult to provide enough
whitening so that the replaced ivory does not show darker than the
others.

Added to this, my experience with intelligent people (read: teachers)
regluing their own keytops includes flats glued on backwards, ivories
on backwards or crooked, and some so misaligned that they hit on
neighboring keys.  And very few people, even technicians, pay attention
to matching the joint of the head to the tail.

Bill Maxim
Simpsonville, SC