David Nebenzahl wrote:
....

I disagree. Although it's difficult to know exactly what the ceiling
looks like, based on what the OP said above--that the differences are in
sheen, not color--it *should* be possible to fix the problem, at least
somewhat, by applying a clear sealer of some kind. I'd say anything
that's not too glossy should work.

....

Well, I said it _might_, but I'm skeptical they'll be pleased and I'd
strongly recommend against it w/o trying it on a sample piece before.

My objection/concern/whatever you want to call it is that while you
_may_ fix the sheen, the clear finish will accentuate the difference in
the pieces appearance owing to the differing absorption so that what now
appears roughly equivalent in color will then not only have the
reflectivity difference but the color differences are then going to be
accentuated.

Again, it's not possible to tell for sure w/o seeing it, but I'm trying
to prevent a worse situation than they already have that would be even
more difficult to try to repair...

It's easy to say "sure, go ahead" when it's not you who will have the
issues to deal with.

I'd far rather err on the side of caution when I know there's a real
potential for it to come out not at all like they expect...

--