I'm not an expert and I'm not sure that I am right, but I do know that this might mean a few things. First of all, snails' shells can change color as they grow up. As snails age so do their shells so a color change is common. However like you said, it changed drastically after you got him. The more likely thing is that your snail is adapting to the new water. Your water might have a different mineral content, pH, temperature, or many other differences compared to the water of the seller. As the snails adapt to this new water, their shells will change colors. Along with this, a new diet will also affect your snails shell. I don't see anything visibly wrong with the snail so these few changes in its environment likely contributed to this change. Also side note, that snail is so beautiful! However, as I said before, I am not an expert and there might be something else here but to the extent of my knowledge, I see the things I pointed out as the likely cause. Lastly, a snail might change color due to stress caused by its environment. Be sure to check the water parameters and make sure they are all good. Also, make sure he isn't being bullies by another tank inhabitant. If that isn't the case and the water is clean and normal, then stress shouldn't be the cause. That was a lot of words but put simply, as long as your tank water is fine and the snail isn't showing any bad signs, its likely just adapting to its new home. Best wishes, Nele.
