"Roan is a horse coat color pattern characterized by an even mixture of colored and white hairs on the body, while the head and "points"—lower legs, mane and tail—are mostly solid-colored. Horses with roan coats have white hairs evenly intermingled throughout any other color. The head, legs, mane and tail have fewer scattered white hairs or none at all."
Step 1:
Obviously, have your drawing prepared. I keep my line art, color base, mane/tail/eye(s), and markings on seperate layers. Each layer is "clipped" to the base color so I don't have to worry about coloring inside the lines or cleaning up when I get out of them.
Step 2:
I scribble or "squirkle" an off-white where I want the roaning affect to be. I will use thicker, more clustered lines where I want the roaning to be more dense. For this tutorial I used a little cross-hatching.
Step 3:
Using the watercolor tool, I "blend." The watercolor tool is easy and if you don't overuse it, makes a nice blending tool.
IF a horse has white hairs on the points, it is very sparse/barely noticeable. I have not encountered a roan horse with roaning over its entire body.
The artwork is not for reference purposes.