Yes, matching helmets is the norm.
That's really up to the squadron leadership. All the Navy
requires
is that the helmets be highly visible, including the use of reflective tape. All so that SAR can see them more easily. Essentially, barring any requirements by the squadron, the pilots are free to decorate their helmets as they wish, as long as the guidelines are met.
Yes, it looks sharp to have the helmets all match in a squadron, but the Navy doesn't require it. Hence, the helmets seen in Top Gun.
Some of them might have been a bit close to the visibility requirement, like Merlin's, but...
OPNAVINST 3710.7U CH 43
8.2.1 Aircrew Personal Protective Equipment Requirements
a. *Protective helmet — The helmet shall be 100 percent covered with
white reflective tape except as modified by approved aircrew system changes.
Up to 30 square inches of light-colored reflective tape may be applied so
long as the white tape remains visible from all directions. The use of
reflective tape may degrade NVD performance. Temporary, nonreflective cloth
covers may be worn over the reflective tape.
Does that help?
Pat☺