Cowry information please!

#1
Hello all. I have two small (about 1-inch long, 3/4-inch high/across) cowries in my 37-gallon marine tank. They are doing great, and seem to be well established in there.

Do I need to buy some slightly larger empty shells for them to eventually move into? Do cowries move out of their current shell, and into new ones?

And if I didn't know to, or just decided not to put empty shells in there, what would they do? Die?

One more question. They are this mottled brown & white color. Do I have to get replacement shells of the same color?

Thanks in advance for any assistance in this matter,

Shane