How do I make a good sabino on a horse? Here's the horse im doing art for h.//horseeden.com/horse.php?id=20784411

After doing some prep on the horse, I'd duplicate the layer of the horse's body. Then erase anywhere on the horse that isn't a main place of markings. After that, fiddle with the hue/saturation/brightness to turn it white. Merge that new white layer to the original layer of the horse, then make another new layer. On this layer, use different brushes to get fade to the Sabino pattern along with any other small marks.
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Im sure there's an easier way, but this is how I would do it.

Thank you! I drew the sabino on a separate layer, messed with the blending options and made 5 more layers.

I'm seconding Grey, that's the way I do white markings too.