Not really -- roan is the mixture of gray or white hairs in any base coat such as black, brown, bay or chestnut. Sometimes a foal will start with a number of dark hairs sprinkled throughout its coat, and those hairs will lighten with age while the base coat color doesn't change.
Not certain it has ever been determined for sure how an Appaloosa loses some of its base coat color over time, although The Appaloosa Project at Appaloosa coat patterns, coat colour genetics and practical information for breeders of spotted horses - The Appaloosa Project
is working on it. (Caution: it is a $20 to $35 per year subscription!) Some say it's a modified graying gene that does not eventually result in white; others believe it's a sabino modifier. Still others think it's a characteristic found only in the Appaloosa, but if that were true, how do you explain that not all Appaloosas have it?! There used to be a Yahoo group that dealt with all horse colors and modifications; that was 1RainbowHorses@yahoogroups.com
-- but it's history now and I don't know offhand of any others, tho' there may be some.
You might try as Google search for equine color coat genetics. It's a wonderful study! -- but a true "science" of equine coat color heredity has yet to emerge. That's what makes it so interesting! -- especially in the Appaloosas!