There was an interesting story in the April 11 New York Times about Hansen's extraordinary color. He's registered as "gray or roan" but to most eyes he's a gray who's turned white unusually early. Grays almost always become white if they live long enough but generally not by the spring of their three-year-old year.
The most famous gray of modern times, Native Dancer, appears no less than six times in Hansen's pedigree, but the color comes from elsewhere. His sire Tapit's dam was gray and her color can be traced back to Alcock's Arabian, foaled about 1700. To be fair, so does Native Dancer's color and that of every other gray Thoroughbred of today.
Native Dancer has become a figure of incomparable importance to the modern Thoroughbred, appearing in the pedigree of almost every successful horse foaled in recent years. If they don't have Native Dancer blood, like AP Indy, they are immediately bred to mares who do. I think if you examine the bloodlines of every Kentucky Derby contender you will find the Gray Ghost. But you won't find his color, as least not because of him. His genetic importance is mostly due to two horses, his chestnut son Raise A Native and his bay grandson Northern Dancer, and they inherited--and passed on--his speed and the ability to carry it a distance but not his coat color.