How Common are Blue or Green Shells

I recently was discussing murex specimens with my friend, and he mentioned the existence of blue murex shells. I know that pink shells are common; how common are blue or green shells? Is the murex always covered with growth that resembles the sand of the sea bed? From what I remember of the snorkling tour I took thirteen years ago, the shells I saw were a green-yellow that resembled the sand beneath them.

The shells, when dried and cleaned are neither pink, blue or green – they are “sandy” colored, with brown stripes. When they are in the sea, whatever grows along the sea floor, grows on them. The snails in Dor are covered in algae which ranges in colors like beige, green and purple. – Mois Navon