I believe the buckskin colored gelding I have is a champagne as I believe his mother was. I did not know her parentage. He was born with bark blue eyes that turned amber and had many Champagne characteristics. I do know that his color did not come from his sire.
I realize that grullo as referred to by AQHA must have a dun factor, but the same color is very evident in many champagne horses.
Have you been to the International Champagne Horse Registry website? There are many horses pictured there that are the exact color of this horse. I think they have around 600 photos in all. This registry calls them 'Classic' if they modify the black gene and 'Sable' if they modify the brown gene. Then, you can also have the roaning seen with the rabicano or any other 'pattern gene'.