This came out as a FB re-post right after I started reading on the Equine Tapestry website myself.
The Bay Horse Post: A common question is “What kind of bay is my horse?” The short answer is all bays are genetically the same even though bay can vary in shade from a light sandy shade to a black bay shade. Every bay horse has the ability to produce black and red pigment at the Extension locus (coded as E) and the Bay allele at the Agouti locus (coded as A) which directs black pigment to the points. Bay is believed to be the “wild type” color in horses which means it is the original basic color of the species. As such, Bay is not a modification of Black, Black is the result of a later mutation at Agouti. For a more in depth explanation of how Extension and Agouti work together to produce the basic colors of horses (Bay, Black and Chestnut/Sorrel) I recommend reading: http://equinetapestry.com/2014/09/bay-is-not-a-modifier/ and follow up blog posts by Lesli Kathman.
*provided by Lewella Tembreull
Here are the pictures that were attached to that post –
There has been an issue with the brown color tests and these horses are now being called dark bays… I know that I will be keeping up on this coloration – as I have had several ponies and horses over the years that fit this brown or bay classification.
and “Wild” Bays –