So, I wanted to know more about this, so all week I was looking at my Cardinals (around 25 of them) and one morning I was able to spot one blue!! As soon as I turn the lights on, it started to get their particular red color (it took it like 3-5 minutes). So, I was right they changed color depending on the lighting. I tried to take a picture but my phone was not focusing. I will try another day.
Also if you notice they have spots which can be blue, so there must be somekind of natural quimical in their body that generates this bluewish colors.
So, this does not mean that he's not right. Maybe his shrimp has something that prevents it to get back to its "real" color.
By the way, I'm planning to get some Harlequins, anyone knows anything about their care? I'm planning to use RO/DI water with 8.5 Sulawesi minerals.
Greetings to all!