Sooty is most likely present in every horse breed and almost every horse shows the effects to some degree.
Black and bay based horses often have dark hairs and patches on the upper part of their body. This dark topline may look like a rather broad dorsal stripe, as is seen in dun horses. This effect is called countershading, as those parts of the horse are shaded which should actually lighten due to the sun. This might confuse predators.
However, sooty may also be responsible for dark liver chestnuts, which are often darkened completely or only from below, resulting in ‘muddy legs’. If it is, sooty shows a different overall pattern on chestnut based horses, than on any black based horses.
Sooty can also work on the mane and tail alone. There are no clear rules. The better rules you think of, the more exceptions you find. Sooty may be an overall term for multiple genes with multiple effects. Which is not unthinkable if you consider sooty may actually work as camouflage on a horse. I.e. protecting the horse from predators.
As no sooty test is available in real life, there won’t be one in the game either. If the genetics are found and you have proof to show us, please notify us so we can implement this in the game!
In the game the colour is inherited as a single incomplete dominant gene. This means you’ll find horses without any sooty colouring, horses with some colouring, and horses with a fair amount of dark hairs. Also, sooty is held responsible for liver chestnuts.
Sooty seems to increase its expression when the horse ages. This becomes clear in palomino or chestnut horses, that may completely darken and become unrecognisable with respect to the original colour. Therefore, foals do not show any sootiness and you may be surprised when you see the adult version!
As the expression of sooty in real life changes with the seasons and the horse’s age, the in-game expression is actually too static. Since we only have 2 images for each horse, a foal and an adult version. Often, players won’t even the notice the sootiness on a horse’s coat, but it may also result in a few surprises.