Grullo is a horse color characterized by a grayish coat with a distinct dilution effect. This color typically features a mouse-gray or smoky appearance, often with a darker mane and tail. Grullo horses can also have primitive markings, such as a dorsal stripe, which runs along their back, and leg barring, adding to their unique look. The grullo color is most commonly seen in breeds like the Quarter Horse and Mustang. It results from a specific gene that dilutes the black coat color, creating the grullo effect. This color is relatively rare, making grullo horses particularly sought after by enthusiasts and breeders.