I have found the same thing. I have birds that hatch white and 3 months later, they are spotting up. I've been holding back boldly spotted chicks and selling off the less spotted but reports are the less spotted, they change to well developed spots later.
Actually I've found the exact opposite with my blue paints. All my paints have been hatching out white and then develop spots as they get older. The blue paint roo I posted that is loudly marked hatched out completely white and developed spots as he aged. I'm not talking about a 1 year old bird being white and then turning paint after a couple years but mine have been hatching out white and developing color in the grow out pens.