Very cool they are purple. It's not as rare as it used to be, but it's usually very desirable, so it's easy to sell purple snails as a rule. The ivory and gold ones are the most common colours. It's the body that determines the snail colour. Shells can be any colour but ivory shells on purple bodied snails look pretty neat.
Really blue coloured snails are the ones I have the hardest time finding around here, but I see them now and then. It can be hard to tell sometimes, whether the snail is really blue or just dark purple.