If a peice of zimmerit 'breaks off', what colour do you expect to see underneath?
Bare metal, primer or base colour?

Hi
Bare metal was unlikely. From what I've read, it was applied over the primered armor.
So you'd see primer if it just fell off, and of course it could be painted if older.

If it's a rebuilt/refitted hull, then there would be a base color of some kind under the zim.

If you look at photos, areas with chipped zim have the same color as the rest of the hull.

Cheers
Andy

if you are chipping away the Zimmerit to represent a hit, you would go to base metal. Other than that woud probaly be primer or base coat as Herberta said