I don't have any experience with WCP either. However, juvenile Pionus of other species do show different colours to adults. Young Blue-headed Pionus can have red feathers at the top of their head near their beak and the blue on their heads is often a more muted colour. Young Bronze-winged Pionus look a bit washed out and their brown wing feathers may be tipped with a paler, more golden brown. It can take a couple of moults for these colours to intensify into their adult plumage.
It would be cool to take regular photos of him under natural light and see how his colours change.
I only have white caps and yes the top should change to white though none of mine have a yellow color there. I wonder if it is staining from formula or bedding or something else. A baby wc pi is so drab compared to an adult. WC pi are my fave tho.