No a horse will not leopard out. They are born leopard or not. Varish will lighten the coat revealing spots (think of spots as a hole in the white blanket covering the horse) that would be visible if they were a true leopard.
So, you're saying the coat CAN lighten like a grey's? Just trying to get it straight - Verona said Varnish won't progressively lighten the coat, but I've heard and somewhat seen differently. I know it can and will change season to season, but realistically I've never seen a varnish horse GAIN color after a winter, just lose it...