Plumage Transition?

It took two weeks for the American goldfinches to find my bird feeder, but now up to six goldfinches at a time are feeding. Goldfinch plumage seems to be in transition, with many starting a patchy black spot on the front of their face. It looks like a dirty face in some photos!  In addition, the pale plumage of winter appears to be undergoing a change to the canary yellow of spring and summer.