There is a huge variety of chromatic harmonicas with enameled covers.
The Emil Friedel company produced some wonderful art harmonicas, so did F.A. Böhm.
The first Hohner Larry Adler harmonicas (10 and 12 hole) came with colored covers. There is also a series of 10, 12 and 16 hole Bandmaster harmonicas with beautiful ornamentation.
The concept of coloring chromatic harmonica cover-plates must have already been around since the 1920th. At that time, there were already chromatic harmonicas with external spring and enameled covers available.