My understanding is the Perculas continue to develop their stripes or patterns later, but Oscellaris don't.
Which are the Picassos?
I had many of the Oscellaris babies for 7 - 8 months and the pattern they had at 1/2" is the pattern they kept. On some of them, the orange would continue to darken and more black on the fins would develop, but the white on them didn't change.