When they are being raised, often they have a really mineral rich environment, possibly optimum water hardness, great diet. When they go to a new home, the water and foods are different and then their shell will change color sometimes lighter and sometimes darker.
So it is a normal occurrence. Just take a look at the new shell, does it look strong
or very thin? What mineral content in the water, especially KH
/ calcium / alkalinity.
You can make adjustments if you notice any issues. If needed you can increase the calcium in the water, feed a shrimp or crab food with the necessary minerals and dietary requirements for an animal which needs to produce a shell.
Not too long ago I had Blue Mystery snails and for some reason the shells became more blue in my tanks. I will supplement them with Hikari Crab Cuisine and have a Cuttle bone in the tank too.
But they eat all foods/ leftovers whenever possible, covering the food with their massive body. Before that I did notice with one group I purchased they had been in harder water before I got them and I noticed shell erosion from my low / almost no KH
tap water (when I had stopped using a calcium additive to see how it went with my pH and tank stability).
Kent's Marine Superbuffer dKH
is what I have used, and it is fairly low priced. I got the larger container maybe 2x the size, for not much more than the smaller container last time.