I had an Appaloosa mare that was born solid black with a star and by the time she was 2yrs..she had roaning over her body and a faint snowcap...when she reached the rip old age of 35yrs she was a total Blue Roan snowcap!!!! out of 7 foals all bred to a chestnut spotted blanket she had : 1.Red roan snowcap, 2 Bay roan frosting blanket ,2 red roan roaned blanket, 1 bay marbled roan ,1 red roan no appy spots , last two foals by a Bay roan blanket she had 1 bay roan snowcap and a Blue roan leaopard.......
So yes they can and most do continue to color out more each year......Never would of dreamed my mare was a Blue Roan snowcap... She died at the age of 37 and was a great 4H horse for 6 years to my neighbor...