*UPDATED 5/7/2015*
I've heard some users complaining about how hard roans are to do. Not anymore! This is my "technique" for making roans. The example above is a red roan, but it can be used for all roans (you just have to use different colors XD).
Please follow the rules found here.
Step One: Color in your horse's base color: bay, chestnut, black, silver black dun champagne - whatever it is.
Step Two: With the pencil tool, set to 100 scattering and somewhere between 20-35 opacity (lower opacity for dark colors, higher for light), go over the horse with white on the neck, body, and upper legs.
Step Three: Raise the opacity by 10-20 (whatever looks better for the color) and do it again, but move further inwards (covering less of the horse to create a gradient look). Repeat until you reach somewhere around 150 opacity, or until you're satisfied.
Step Four: With the blur tool, set to 0 spacing and 60 or less opacity (depends on what you prefer), brush over the horse once. You can brush lightly with the airbrush (in white or base color) on the edges of the roaning to blend it a little more if you like. And you can even use the airbrush (in white) over all the roaning to give it a smoother look, if that appeals to you.
Step Five: Clean up the mane and tail and, if you choose to do so, add corn marks.
Step Six: Add markings/patterns/etc. of your choice and viola! You are finished.
Blue roans have black bases, and red/strawberry roans have chestnut bases.
Examples of Roans I Have Created Using this Technique
Blue roan
Buckskin roan
Strawberry roan
Blue roan