Many creatures sport beautiful colours even though they have little or no vision themselves. But why?
Well, camouflage might be one reason. In the case of some molluscs iridescence, related to the crystalline structure of the shell, is another. In the giant clam, for example, the mantle (the layer beneath the shell) plays host to colourful photosynthetic algae, which trade nutrients for the protection offered by that mighty mollusc.
But the colours of some shells are forever hidden on their undersurfaces, or only become apparent after death; and many colourful shells spend their lives far from view, buried in sand or mud. In which case, we can truly say that the reason why some shells are colourful is a mystery.
It is also a salutary lesson for us humans, who are always keen to find an explanation for adaptation in terms of our own senses. It could be that the colours of shells are incidental by-products of some other process, not there to satisfy human aesthetics.