There's a stamped steel plate with a series of numerical codes on the driver-side of the engine compartment.
On the second line is a series of numbers followed by either a "G" or "H." The first series of numbers is the body; the second number is the hardtop. (180G is silver grey metallic, for example.)
You'll find a key for the paint codes in the database section of this website.
(This information will only be helpful if your hardtop and its paint are original to the car, of course.)
Douglas Kim
New York
USA