I decided to do one of these
Step 1 - get your base done first. Then add a layer and pick the color you want your roan base to be (usually a light grayish tan if a brown base) then color the area with that color where you want the roan.
Step 2 - either using the airbrush and or the soft eraser, soften/blend legs and neck area or all the hard areas of that roan base.
Step 3 - then using either the pen or pencil put it to around 1-3 in size and max both the spacing and scatter. I usually have the opacity between 30 and 80. Then take that and do circles all around the edges of the roan with that roan base color, to get a bit of a speckled look.
Step 4 - doing the same thing as step 3 but with a white color and push alpha lock and do those circles around the whole roan base. Fill the roan base as much as you can, try not to go over your previous circles as much.
Step 5 - repeat step 4 but with a darker colored version of the base base color
Step 6 - I usually will go over the whole thing with the roan base again repeating step 4 but with the roan base color. Then I airbrush over parts or the whole thing with the roan base color. You can then choose if you want to use the soft eraser and erase by the soft parts of the horse or make the whole base less transparent.