Which horse color has the least distinct pigmentation with shades of red and black?

The bay horse is characterized by a coat that is a reddish-brown color with black points on the mane, tail, and lower legs. This color mixes shades of both red and black, which often leads to a less distinct pigmentation when compared to other horse colors.

In contrast, the chestnut horse exhibits a solid reddish color without black points, displaying a more straightforward pigmentation. Grey horses, while they can appear in various shades, typically start as a colored horse that then lightens over time, making their pigmentation unique throughout their life. Black horses have a uniform black coat without the reddish hues present in bay horses.

The bay horse, therefore, represents the blending of red and black in its pigmentation, making it the option that exhibits the least distinct pigmentation in terms of separation between these two colors.