Shells brightly coloured/patterned to attract mates?

[ennss]

Why (in a biological and evolutionary sense) are shells brightly coloured/patterned?

Birds and lots of other creatures use colour/pattern to attract mates, flowers apparently use it to attract pollinators, but why are shells colourful?

Is there much or any sexual dimorphism in shells?    I don't really know,  but I don't think there is - so colours/patterns wouldn't be to attract mates (such as in birds) - also, I don't expect that molluscs have the sensory equipment to see the colours anyway.  (Does anyone know if snails can even see colour?)    I expect that some of the predators like cones must have pretty good motion detecting equipment, but can they detect colour?

It's understood that the shells immediate mineral environment can heavily effect the colour but what about the pattern, is it just random?

Any insight appreciated!