Amano Shrimp Turned Black

A week or more ago I bought 2 Amano shrimp to combat some Diatoms. Since then they have been gorging themselves on it. That being said they do it at night and I don't see them all that often.

This morning I come into the office and I see one up by the intake on my Spec. At first I thought he was dead but I approached him with some tongs and he scurried away into hiding. The odd part was that he was dark or almost black in color. When I bought him / them he was clear. Is this an aspect of their diet, protection, molting? I'm new to fish, planted tanks, and shrimp so just wondering what's going on.