Sounds like a spawning occurred at some point. I am assuming the
original four fish have grown somewhat (25 cent goldfish can reach
reproductive size their first year), and the two new goldfish are
somewhat smaller. Young goldfish are usually brown, and take on
orange as they grow . Keep an eye on it: soon you may have 7
goldfish and no oddballs.

"anon" no spam wrote:

I started in my small pond with 4 goldfish ( just the basic .25 cent
variety). Now I have 6 goldfish and 1 oddball that is the same size and
shape as the other small goldfish but it is a different color. It is a
murky brown/ black with a little light orange down its back.

My question is: Do goldfish have on occasion babies which are a different
color than the normal orange? If not, I have no idea where he came from.
There is no pond or stream nearby for him to have wondered in by mistake.