When Do Baby Goldfish Turn Gold?

A goldfish's color responds to light, and does not depend on age. If a fish is kept in the dark it will get lighter,  losing much of its color over time. For a true gold or simply your fish's brightest, healthiest color I suggest giving it full spectrum light. As they are outside, you are already doing that with natural light.

Diet plays a role in color, shrimp and fish meal will also help bring out a goldfish's color.

However, color variations that you are describing seem to be more along the line of the fish's dna rather than anything that can be changed with time, light or diet. I think you simply have dark fish.

I've never seen gold goldfish. Only orange-colored ones...

It depends on the light