Grulla isn't a breed, it's a color, resulting from the effects of the dun gene on what would otherwise be a black horse. Dun is considered a dilution gene. Dun factor horses have lighter bodies, with darker color on the mane, tail, and legs. Nearly all duns have a clear, distinct darker line down their spines, referred to as a dorsal stripe. They often have other "primitive" markings like zebra stripes on the legs, one or more cross stripes on the shoulders and/or neck, and dark "cobwebbing" on the face.
Duns come in many shades, depending on what other color genes the horse has. Dun turns a Chestnut into a Claybank or Red Dun, a Bay becomes a Buckskin (some call it a Dunskin, to differentiate it from the similar color resulting from the action of the cream gene), and Black becomes a Grullo. Grullos come in many shades, from a sort of silvery gray to a sort of medium mouse brown.
Dun genes occur in many breeds of horses, from Mustangs to Minis, but one breed that only comes in dun colors is the Norwegian Fjord. In Norwegian Fjords, the dun factor dorsal stripe results in a two-tone mane, which Fjord owners often show off with creative trimming (the horses in Disney's Frozen I and II are Norwegian Fjords).
Perhaps the reason you got thinking about roan is that true roan colored horses are typically lighter on the body and have darker legs and faces, but the reason for this is different. Roans have white hairs mixed in with dark hairs on their bodies, with few or no white hairs on their legs and faces, while in Duns, the body hairs themselves are lighter in color. And, just in case you weren't already confused by this blizzard of words, Dun and Roan are caused by totally unrelated genes, so it is possible to have a horse that is
both Dun and Roan in color.
