Many years ago one would occasionally see sable Australian Shepherds, however it is very unlikely that the gene still exists in the breed gene pool today. The sable gene is dominant to the tan trim gene. Most Aussies have tan (copper) trim and therefore cannot carry sable. Sable has been a disqualification in the Australian Shepherd breed standards at least since the mid 1970s. It is most associated with Rough Collies and was never considered typical for Aussies.
Aussies people sometimes think are sable are actually yellow. (The shade can vary from deep chestnut to pale lemon.) The gene for this color is separate from the one that produces sable. It is recessive, so any time yellow pups are produced, both parents carry the gene. Yellow Labradors and lemon Pointers have the same gene for yellow, but in those breeds the color is allowed.
In sable dogs, the hair is banded – the tip will be black (or liver if the dog has liver pigment on it’s nose, etc.). Yellow hair is the same color throughout. The color of the nose is not an indication because the black/liver coloration is controlled by yet another gene.
Yellow dogs will not have tan trim, no matter whether their parents had it or not. White markings are not affected.
While both sable and yellow are attractive colors, they are disallowed in the Aussie breed standards and with good reason. Both can mask the presence of merle. Because of this, one wouldn’t necessarily know that a dog was merle and might breed it to another merle, producing puppies that are blind or deaf because they have two copies of the merle gene.