Hi there! I believe you have mystery snails, which is a type of apple snail! They are constantly growing their shells, and they need a supplemented source of calcium carbonate. If you find that they are losing the color in their shell, it is most likely due to a lack of food/calcium. It takes A LOT of food to grow a beautiful shell! Try adding some veggies in there. You can prepare veggies by washing them well and taking the skin off.
Some people blanch their veggies, but I find it unnecessary. I tend to prepare a lot and freeze the pieces. Make sure that the snails have food EVERY day. As mystery snails get to their full size, they eat a TON! I use a simple rule of thumb: try to feed what they will eat while I am asleep. I feed right before bed, and I try to make sure that there is no food when I wake up. Veggies I keep in the tank 24/7. Sometimes it takes a day or two for them to eat all of it. Some good veggies to feed are: cucumber, squash, zucchini, or broccoli. They LOVE lettuce and spinach, but they eat it very quickly.
I also make snello. If you like, I will be happy to write up a post about how to make it, what I use, and why it is so important for snails. I have to make some on Sunday or Monday, so I can take pictures as well. Snello is basically a veggie rich gel food you make for plecos/snails.
A snail's shell tells a story. What story is your snail telling?
Hope this helps!