Not necessarily I am afraid- I agree that they are visually pretty much white but I know of at least two, true, proven by breeding, Fewspots that were born a sort of solid- one was chestnut at birth but faded within three days to white with just a hint of red in his mane and tail, most strange, the other, who is still alive, I think, was, as far as I can ascertain, born solid black(!!) I did not see him then but by the time I did at three years old he was a typical Fewspot and has never sired a non Appy foal out of any type or breed of mare he has been put to, since the last I heard he was 25+ I think we might assume he is/was a Fewspot.
I am also pretty sure that both parents have to have the Appy gene but do both parents actually have to be visual Appies??