Here is a link that may explain the genetics that cause it. I am horrible at the genetics that make up the alphabet soup it makes my head spin. But it does have to with the type or lack of black-melaniser genes present that cause it. It also why some blacks look muddied or brownish and some are...
Pullets can have leakage it’s not as common but does happen. I have a satin that has a few feathers that have gold. In the picture you can see it in her hackles. I was surprised to see the gold feathers when she was feathering out because her father was a silver birchin.