Guppy fish help

[Clangille14]

So I've had a 10 gallon tank for a while that has 3 guppies a couple gouramies and some mollies. i recently switched over to a 30 gallon after letting it filter through for a week. All of my fish seem fine except my one guppy has some red striping around its head and its tail has turned black, not its fin, the part that connects the body to the fin. He seems to be swimming fine and eating fine its just worrying me that its turning black. Theres no white fuzzy ness around it i don't believe its fin rot. Could this be stress?

 

[Clangille14]

Here's what he looks like he swims to fast to get a good pic

 

[Slamatrix]

I have a guppy that is similarly coloured, dark silvery with yellow fins etc. Mine also changes colour quite regularly, goes to a dark blue quite often then back again to yellow. Not sure exactly what causes it, but i'm sure its nothing to worry about, if it was something wrong with the water i would expect to see more fish affected. Plus i also have an upside down catfish that changes colour during light changes (at night its very light in colour, then during the day its darker). I would keep an eye on its activities, if it starts hiding, or staying still at the bottom, anything out of the norm i would worry. But if its swimming around normally with its friends i'm sure it'll be alright