Cupcake has one yellow feather

George was something of a contortionist and would twist himself into some odd positions to be able to preen the pin feathers on top of his  head. Sometimes he would rub  his head on the cage bars too, so that's probably how the feather on his yellow crown got damaged. Ralph depends entirely on my assistance with the pin feathers on the top of his head, much like Cupcake does. It's possible that something may have damaged the feather on her head before she joined your flock-- maybe when she was fledging. That's not uncommon, and seems more likely to me than any harm being done by your preening assistance. I've read that it may take several molts before an odd-colored feather will show up after a follicle was damaged.