The dark brown shades of bay are referred to in other languages by words meaning "black-and-tan."

Some bay horses exhibit dappling, which is caused by textured, concentric rings within the coat.

The red areas of a bay coat usually have a two-toned hair shaft, which, if shaved closely (such as when body-clipping for a horse show), may cause the horse to appear several shades lighter, a somewhat dull orange-gold, almost like a dun.

Some shade variations can be related to nutrition and grooming, but most appear to be caused by inherited factors not yet fully understood.

Wild bay is sometimes found in conjunction with a trait called "pangare" that produces pale color on the underbelly and soft areas, such as near the stifle and around the muzzle.

It is clearer to refer to dark-colored horses as dark bays or liver chestnuts.

To further complicate matters, there apparently exists more than one genetic mechanism that darkens coat colors.

The other is a specific allele of Agouti linked to a certain type of dark bay, called seal brown.

Some foals may be born with a few white hairs already visible around the eyes, muzzle, and other fine-haired, thin-skinned areas, but others may not show signs of graying until they are several months old.

At agouti, the dominant, ancestral A allele limits the location of black pigment to the points, seen in the bay color.

A genome wide association study identified a region of equine chromosome 22 that appears to correlate with the extent of black pigment on bay horses.

Over 42,000 years ago, a mutation called non-dun 1 appeared, which allowed horses to be bay.

Later a second mutation to the dun gene, called non-dun 2, was able to remove the primitive markings altogether to create the non-striped bay color common today.