In addition to chestnut and black, bay is a third base colour. It comes in different shades and may have many names, but the general term is bay. Brown may also be used.
A bay horse always has a reddish coat with black 'points'. These points are the outer edges of a horse, namely the mane, tail, lower legs and ear tips. In a bay horse, these are all black, or at the very least, a very dark shade of brown. Due to other genes, like cream or silver, the colour of the coat and points may change.
Bay is the best known variation of agouti. The agouti gene takes the black pigment and moves it across the body of the horse, resulting in patches where red pigment can 'reappear'. It can therefore only work on black pigment, and as a result needs at least one copy of the E-allele to function.
Agouti is the gene that is responsible for bay or brown horses. Agouti comes in many different shades, from a very light mahogany bay, to blood bay and even dark brown. The presence of agouti (in general) can be proven by a laboratory test. It is not yet clear whether the different colour variations of agouti (phenotypes) have a genetic base or are created by environmental factors. For that reason it is impossible to test if a horse with agouti is any specific shade.
There are three versions of agouti commonly accepted, but more may be present.
Bay is a common colour in the game and appears alongside seal brown and wild bay (least common). As we had to make a decision in the past, all these shades are different versions of the agouti gene.
The available test in Horse Reality tests for the presence of agouti in general. If present, it is not ascertained if the horse carries bay, wild bay or seal brown. Just that at least one allele of agouti is present, not to which version it leads.