Please explain how shell dealers grade shells and where these grading standards came from . Someone may indeed varnish a shell, but this is not the usual way.
Some mollusks cover their entire shell, others only a part.
Since the mantle that covers the shell is also the part that actually manufactures shell, it can add tiny amounts of shelly substance as it enfolds the shell, smoothing and shining it, keeping its surface free of growths and encrustations, and from dulling by chemicals in the water. The shells are not, as is often thought, painted or dyed. The colors are sometimes determined by heredity but in some species can also be influenced by the diet of the animal. Color in mollusks often serves as camouflage, but some pigments are primarily structural in function, serving to strengthen the shell. The yellows and reds of beta carotene are an example.