But he shouldn't be getting more yellow! He's a pied! A light pied, but still, 100% pied, definitely not split for it. Here's a pic of him showing off his wing (which is all funked up due to a growing out clip and current molting)

I realized today though, when he got all skinny due to a sound he didn't recognize, that his face looked a lot more grey. So maybe his yellow feathers are somehow underneath the grey? I dunno.