Hi! Not sure if they would accept silver dun....I had a silver buckskin filly 2 years ago and tried to register her as that and they told me the color didn't exist and wouldn't let me register her as that (I had her tested before and it came back as silver buckskin!). SOOO I'm guessing they probably wouldn't accept silver dun but you could always call and ask! I know it's a pain in the rump but it's easier then sending registrations back and forth.