I assume Killarney is referring to the topsides and not the deck. In theory, fiberglass never totally cures resulting in the possibility of hull deformity but not sure that's really true given the myriad of top-of-the-line (read most expensive) boats built in just this way with either dark colored gelcoat or dark painted hulls over Airex cored innards. In fact there are virtually no contemporary boats built with other than hulls cored with something, often Airex. I wouldn't worry too much about either a new or older boat constructed that way as the new one will be guaranteed and the old one would have long-since demonstrated any ill effect.
Although a matter of personal opinion, I don't think there is anything prettier than a sailboat with good lines and a dark hull color.