Given the comment thread, it seems safe to offer this reparsing, owed entirely to Greg:

un parfait « arc-en-ciel » → un parfait couleur arc-en-ciel

There are various dishes called a « parfait », but the dessert glacé (also see more detailed English version) seems like the most likely one.

Searching for « parfait arc-en-ciel » suggests that the rainbow could be achieved either by dye or by using something like your fruit hypothesis to constitute the « parfum » of the parfait.