Clownfish turning black?

[Miami Reef]

I bought 3 clownfish from lfs marked as true percula about 1 and a half months ago. Out of 3, one of them is turning black. You can see the comparison. The clown fish looked like the orange one when I bought them. What does this mean?

 

[Crabs McJones]

I bought 3 clownfish from lfs marked as true percula about 1 and a half months ago. Out of 3, one of them is turning black. You can see the comparison. The clown fish looked like the orange one when I bought them. What does this mean?

My original ones did the same. Interested to see some of the answers.