I had an appaloosa that was born bay with one small spot on his rump. When we got him at one he had little white hairs on his withers. When he was two he roaned out. Before I sold him he was more white than bay.
He will definitley roan out like his momma. As a baby, his coat was very solid, but if you look on his back, you can already see white hair. My boy was born sorrel with a white blanket, but his momma was a blue roan QH, so he is getting more and more roan each year. And it wouldn't surprise me that of he gets a scratch, the hair will grow back white, as on my boy as well.
I've seen appy's born bay with just a few white and black spots on the rump go white with black and bay spots and a black and white streaked mane and tail. Didn't start going more white till she was 3.