How rare is a Silver Seal Brown Cream Appaloosa?

I just bred one and have no idea if it's rare or not or if it'll show up differently once it is older.

Well, based on the horse search, there are 807 active, unlocked Appaloosas.  And only 6 of those are silver seal brown creams.  So pretty rare!  :)
The color won't change as the foal ages, though it may look a bit different on the adult art as compared to the foal art.

pretty rare.