Which color is described as mousy or smoky?

The color described as mousy or smoky refers to the grullo color. Grullo is characterized by a horse coat that has a grayish or smoky appearance due to the dilution effect of the cream gene on a black base coat. This results in a coat color that can present as a mouse gray with a subtle hint of a brown or blue hue, which aligns with the descriptors "mousy" or "smoky."

Conversely, the other options represent distinct and different colors. Black is a solid, deep color without the muted tones associated with the grullo. Buckskin, characterized by a tan or gold color with a black mane and tail, does not exhibit the smoky quality either. Red Dun is more of a reddish or tan color with a dorsal stripe, which clearly diverges from the mousy or smoky descriptor. Therefore, grullo is the only color among the options that accurately fits the characteristics provided in the question.