I know this sounds gross but take him out and smell him. If he stinks (it'll be a very obvious smell, nasty!) he's dead. Then you want to wash your hands fast because the smell is so gross! I hope he's alive for you.
Be advised i've read snails can lay dormant for something like 2 weeks, a good sign if he's dead though is his shell will open up. If its closed and secure, or it goes in when you pull him out of the water he might still be alive.
Yep exactly...just cuz he hasn't moved doesn't mean a whole lot. You dont have to go through the whole "take him out and sniff him" if you stick a net in there and he moves
As for the color...I noticed that with mine the bigger they get the whiter they usually get. They have smoother shells and more vivid colors in tanks that have a little lower pH or perhaps have plants in them and some more nutrients in the water (they always turn almost white in my cichlid tank for some reason...and I'm guessing its the higher pH /hardness and/or no plants to eat)