"Of all colours presented by the horse, none is so rich, and, at the same time, so elegant and chaste, as a bright bay; provided the mane, tail, and lower parts of the legs, be black. A small white star on the forehead, and a white speck on one of the heels, are to be considered, rather, as beauties, than defects: but much white, either on the face or legs, whatever be the general hue, is quite the reverse of desirable. After bright bay, chestnut, perhaps, deserves to rank next in the scale of taste; provided it be not, as is very frequently the case, accompanied with white legs. Some of the various shades of grey, however, are, in the opinion of many, entitled to be placed above it: of these, the silver grey, with black mane and tail, claims the highest place. Brown is rather exceptionable, on account of its dulness. Black is not much admired; though, as we think, when of a deep jet, remarkably elegant. Roan, sorrel, dun, piebald, mouse, and even cream colour (however appropriate the latter may be for a state-carriage-horse) are all to be eschewed."Young Lady's Equestrian Manual
In a late 15th century veterinary treatise, Proprytees of Medicynes of hors (1496), the author talks about the properties (qualities) of the best horses from all countries and judging a horse by its color. I find it interesting that almost 350 years separate these two texts, and some of the ideas are still the same:
To chese every hors by his colour/
iij.The best colour of ony horse is a blacke baye w a gylt mouth and under the flankis the same colour up to the nauyll/And that his lyppes be ful of riueles/for y (this) is a sygne he shalbe a fyers hors/For to knowe the bewte of this hors: loke that he have a white stere in the forehede or a white feder on the nose/ or a white fote behynde/and a lytill lene heed/short erys/open nosethrylles. and gret eyen and a brode forehede/longe at the reyne/a thynne maane. Brode brestyd. and syde braunded/lene knees and a brode thynne legge/gret synewes/short pastron/syde rybbyd/ and a brode holow dove fote. Shorte loyned/ and a longe skete/and smale stones in his codde/And y he stonde upryght on alle foure fete one ayenst another/And thyle best the best proprytees that ony horse maye have/
To Choose every horse by his color (Section III).
The best color of any horse is a black bay with a gilt mouth and under the flanks the same color up to the navel. And that his lips be full of rivuls (wrinkles). For this is a sign that he will be a fierce horse.
For to know the beauty of this horse: look that he has a white star on the forehead or a white feather (snip) on the nose. A white foot (sock) behind, and a little, lean head, short ears, open nostrils, and a large eyes, and a broad forehead, long at the rein, a thin mane. Broad breasted, and side branded, lean knees, and a brode, thin leg, great sinews (tendons), short pasterns, side ribbed, and a broad, hollow. dove foot. Short loined, and a long skete, and small stones (testicles) in his cod. And that he stand upright on all four feet, one against the other, and these are the best properties that any horse may have.One will note that the author states that a dark bay with a gold mouth with minimal white markings is nearly identical to the desire for a "bright bay" for a lady's mount.
The 15th century treatise goes on to list the rest of the horses by color. a sorrell with white mane and tail with a walle eye on the "off side", a dapple gray with black dapples on his legs and swirls on both sides of his neck, a dun horse with a black stripe, a white horse, a mouse dun, a black horse with a white snip and a white fore leg and a white leg on the off side, a coal black horse, iron grey horse.