Depends on zygosity. First, the stallion needs to be tested for OLWS or you run the risk of a lethal white baby--this is VERY important.
Color-wise, you run approximately a 50% chance of some kind of overo spots, but probably a little less than that due to variations in expression. "Silver" is just a color description in this case--the stallion, I'd be willing to bet, is just a regular grullo. Foal could be black, grullo, red dun, or sorrel.