Which color is described as "the color of a true black horse"?

The color described as "the color of a true black horse" is indeed black. In equine terminology, a true black horse has a solid black coat without any brown, chestnut, or gray undertones. This coloration is characterized by its deep, uniform appearance across the body. True black horses can be distinguished from other dark-colored horses that may have markings or faded areas.

Chestnut, gray, and bay refer to different horse colors that do not fit the description of a true black horse. Chestnut typically refers to a reddish-brown color, gray denotes a color that can lighten with age and often has a mixture of white hairs, and bay is characterized by a brown base color with black points (mane, tail, lower legs). Therefore, the only option that accurately fits the description of a true black horse is black.