“that among the Houyhnhnms, the white, the sorrel, and the iron-gray, were not so exactly shaped as the bay, the dapple-gray, and the black; nor born with equal talents of mind, or a capacity to improve them; and therefore continued always in the condition of servants, without ever aspiring to match out of their own race, which in that country would be reckoned monstrous and unnatural.”
A white horse is not too far different in color than a dapple-grey horse, a sorrel horse is just a redder horse than the brown "bay" horse, and iron-gray horses are just lighter black than a "black" horse. Gulliver's point here is that although the Houyhnhnm society would point to these specific colors as representative of their potential or merit in character, it is silly to assume that one "horse" is better or worse than another if their color is hardly different than the ones they use as scapegoats.
He makes a point here about how race is often used as a means to discriminate and ignore someone's potential even though the only real difference between people is appearance.