It's not progressive. Fewspots are born white, and stay that way. They don't get dappled or fleabitten, and their skin is usually pink or mottled.
Gray horses are born any number of colors depending on the base color genes present: bay, chestnut, tobiano spotted, or even, theoretically, fewspot Appaloosa. But the dark coloration of the coat will gradually be replaced by white hairs (and frequently reintroduced as fleabites) as the horse ages. It's a dominant gene, requiring only one copy to have an effect, while fewspot coloration is indicative of homozygousity (two LP genes).
Grays are FAR more prone to melanoma than other horses, though it is much less aggressive and less likely to be fatal in grays than in other colored horses. Fewspots are more prone to squamous cell carcinoma due to the pink skin around their eyes, gentitals, etc. They are also prone to progressive blindness (from cancer or uveitis, an Appaloosa curse), and have night blindness.