Worried about one of my guppy's, odd symptoms but hes acting fine!?

[TomW]

So my tank is starting to look amazing for what I want it to be, I have a 40L tank with 6 Phantom Tetra, 4 Guppies, and 3 corydoras.

However, one of my guppies is displaying some weird symptoms that I can't find anything online about. On its sides towards its head turns blackish every so often and then he goes silver again and keeps changing between the two. The black can appear in 5 minutes with it also being able to disappear in that time frame too. This is the only one displaying such symptoms. I've checked the water and everything seems correct. He is also behaving normally, the same as the other guppies and eating well. I have been treating the water with disease clear just in case. Any ideas on what's going on?

Here are some pictures of the 2 different colour profiles of the same boy, he's called Don Salamanca.

 

[Colin_T]

He is probably showing off his colours to impress the girls or intimidate the boys in the tank. When they fight, their eyes go dark too. See if his eyes turn black at the same time.

As long as he is eating well and not showing any other symptoms, don't worry about it.