The Pop Quiz Answer about dorsal stripes (Hands On, EQUUS 444) describes this marking as a “dark brown” stripe running along the spine. The color of the dorsal stripe is not universal: it depends on the horse’s base color, as well as what other dilution genes may be present.
For example, a grulla (which results from the action of the dun gene on black) has a black dorsal stripe. A red dun (a dun gene combined with a chestnut color) has a darker red dorsal. Things get really interesting when the dun dilution gene is combined with other dilution genes. A red dun horse who also carries the cream gene can have a very pale golden-colored dorsal. Duns combined with other dilution genes also have dorsal stripes that are darker versions of their coat color. Laura Behning Covington, Georgia