Color changes.

Question
Hi, Okay so one of my male guppies well he changes colors, what i mean by that is that sometimes he makes dark blue colors on the sides of his body , and I'm not exactly sure what he is doing, is it some kind of thing he is doing when he wants to breed?
or just something to show he wants to show dominance since i have another male guppy but the other one is a bit larger then regular guppies that i have in my tank.

Answer
hello,
there are a variety of reasons why this could happen, but since you didn't mention anything else or anything behavioral i wouldn't worry about it.
the way melanophors work in fish scales is pretty simple. All of the pigments and melanin are concentrated in a central node. when a certain trigger comes along (excitement, fear, stress, water conditions, breeding etc etc), the melanin is released resulting in the blue color. Therefore, i think it's safe to say everything you listed is true, with the possibility of other factors, like temperature fluctuation.