While some appaloosa spots have mapping from dark skin, the majority of the edging effects on appaloosas are from mixing white and colored hair.
In the previous post about mapping and appaloosa patterns, I talked about how the skin under appaloosa patterns is generally darker than the white in the coat might suggest. It is not surprising then that mixed hair color mapping, rather than dark skin mapping, is common with appaloosa patterns. That's not to say that appaloosa patterns never feature dark skin mapping. The mapping in the photo above comes primarily from white hair over dark skin.
You can also find examples that have both dark skin mapping and hair-mixture mapping, like this horse.
This area—the"blanket" portion of the pattern—is also where it is most likely to occur. However, mixed hair mapping is more common. For an artist painting appaloosa patterns, this type of mapping also has a lot of notable quirks as well as really interesting variations.
Generally, the area covered by mapping (no matter which kind or which patterns are involved) is less regular in width than it is often portrayed by artists. When it comes to appaloosa spots, though, there is an interesting tendency for the borders to radiate outward much like we think of as happening with "starburst dapples." To accurately capture this effect, however, it is important to note that it is the borders of the halo that radiate. The hair that makes up the halo still goes in the direction of the coat.
Here is a more extreme example of the mapping border flaring outward around a spot. Notice, however, that the rest of the spots in the pattern do not have that same exaggerated starburst. Some have more subtle variations in border width while others are more even. (This is the other side of the second horse in this post.) This is what I mean by appaloosa mapping being more "chaotic" than the typical pinto mapping. Even from spot to spot, there is a lot more variation.
It is not just that the mapped area around a spot or the white pattern is uneven. Sometimes it doesn't exactly correspond with the outline of the spots. The mapping on this buckskin leopard is a great example of how this can given the pattern a more chaotic look.
Misalignment of the mapping and the spots is also visible on this pattern, where some of the roaned halos appear shifted to the left in the picture, leaving white borders to the right of some of the spots.
Sometimes appaloosa patterns have areas were the mapping no longer "outlines" the spots or the pattern borders, but just fills in random areas between the spots or the spots and the pattern edge.
Differences in density and second halos
Another interesting effect occurs when mapped spots are set inside an area of the pattern that is itself ticked with color. With these appaloosas, the differing densities of the two ticking patterns can give a striking visual effect. In the photo below, the very dark mixture around the spots stands out against the more sparse black ticking on the forehand.
Also notice that the spots at the center of his shoulder are more faintly mapped than the spots on his barrel. Sometimes haloed spots on a ticked background have a second halo of white hair. The effect is really striking. The Appaloosa stallion CS Arrogant Playboy has gotten a lot of attention recently for having a particularly dramatic version of this kind of mapping.
I have a few more photos of appaloosa details to share in a third post, and then I am going to switch gears and talk about some recent discoveries in genetics. So more pictures, then some more genetics-wonk stuff!