Many fish change their colours according to their surroundings for camouflage purposes. What type of fish are we talking about here?

How long have they been in the aquarium?, and is there enough cover for them to hide and feel secure?

It might be stress related if they haven't had enough time to settle in to the tank, and/or keep getting distrubed.

e.g. some fish ( e.g. Kribensis ) are renowned for looking brown and drab in the aquarium shop, where they keep getting disturbed and are often over exposed, but colour up nicely at home after a few days/weeks in a nicely setup aquarium.

Far too little information, sorry!

What kind of fish?

Do you have any idea of their sex or territorial habits?

Is it all the fish, or just a few of them and in general, how many fish are we talking about, a pair or a school?

What country do the fish come from?

What country do you live in and how cold has it been outside?

Are the fish being better fed indoors than outdoors?

Are you sure it is not a matter of feeding, lighting and so on?

In general very many fish, especially including aquarium fish, change colour in various ways for various reasons at various times. If the outdoor water is poorly lit, and it is a fish that likes lots of light, that might explain it. If your tank is lit overnight it might fool them into thinking that it is the breeding season.

As you can see, there is wide scope for speculation!

Cheers,

Jon