I would avoid hermit crabs with painted shells, to be honest. The paint often will chip off and end up eaten/absorbed which can obviously cause serious problems. I've even heard of ceramic paint being used...which contains lead if not heat-treated.
The paint can also cause irritation to the poor crab who sometimes can't even get out of the shell (remember that these crabs are usually forced into them).
They seem to prefer natural ones...hermits in painted shells will usually switch to a natural one when given the choice. I have seen this happen myself on numerous occasions.
Hermit crabs are not the simple pets they are made out to be. You'll need at least a 10-gallon aquarium and you shouldn't have just one crab since they are social animals. Try to get at least two or three. They need heat (so you'll most likely need a heat lamp or under tank heater), high humidity (you'll have to mist the tank with dechlorinated water on a regular basis), and a varied diet (a hermit crab pellet food alone isn't really enough).
Some species need access to saltwater and all enjoy it. This means buying marine salt mix. Most tanks have a freshwater pool and a saltwater pool.
As far as diet goes the most important thing is that you keep it varied. Hermit crabs don't like to keep eating the same foods and will sometimes flat-out refuse to eat them. Here's an article: http://exoticpets.about.com/cs/hermitcrabs/a/feedhc.htm
Keep in mind that hermit crabs are wild-caught animals that have been under a TON of stress before getting home. It's not unusual for them to get sick and/or die even with the best of care. If they do recover, though, they can easily live 10+ years.
Here's another website: http://www.crabbytalk.com
They're really great pets, but sadly pretty misunderstood.