Gold fish can come in a lot of colors. White, grey, copper, red, yellow, black along with the classic orangey-gold. And some just stay the original plain carp brown.
But the VAST majority of them start out copper or brown and then go to gold, starting at the belly and fading up. Sometimes as it fades, the brown gets nearly black and is very striking looking. From the pattern on the little guy above, he is fading.
Most (but not all) colored ones also start out coppery brown. Red and white often start out very dark. This is a survival trait. Fry that start out with the bright, pretty colors get quickly eaten by their parents and faster-growing siblings.
This is why when people buy them for ponds, they generally want them 4 inch body length or bigger. By then the color has stabilized.