Is this normal in fish?

I have a mosquito fish(minnow). I got it a week ago it's belly wasn't blue when I got it ,but now its belly is blue. I have 2 minnows do you think the other fish mated with it my other fish is only 1/4 inch and is also a minnow ,but this one is 1''. Could their be something wrong with it because the other fish still has a gold belly.