I've got a bit of experience with partridge and I've never been able to tell until about 6 weeks whether blue or gold. You need some decent primaries and some breast feathers to be sure of what you have as far as color goes. When the primaries are in, if the bottom feather is black, it is probably a gold. If blue, it is probably blue. Pullets you can tell a little earlier sometimes by looking at the breast feathers, If they look more gold than brown, generally that means blue. Both blue and gold will have gray down, the darker the bird the lighter the gray will look. See the back end of this gold partridge hen. The cock is the same, but hard to see in this picture: