I have a pond with koi carp and goldfish. Late last year I noticed several baby fish. They survived the winter and are dark in colour (they do not look like any of the other fish), I guess this for their survival, but when will they change colour?
Chances are EB that your baby fish are goldfish fry. There's no definate time scale when they will change colour. I have seen 1" baby goldfish as orange as they come yet i've also seen 4" goldfish that are still brown or black.
Generally, it's environmental conditions that start the change off....temperature is one factor. Just enjoy your babies....i'm sure you'll probably have more from this years spawnings.
As Hammerman says there is no time scale. A couple of months ago I mentioned I had some fry in pond to my sister who asked for 2-3 for an aquarium she had just bought. This led to several requests for fish which I duly provided, however I'm now being accused of favoritism as some fished changed colour in days and others are still as black as the day I caught them