So I've been without a computer for many years now and do everything on my phone and laptop. When I used to do art, I did it on my phone on this app called MediBang and I stoped doing it because I could never figure out how to add markings to horses such as frame, sabino, etc.

With medibang highlight the part you highlight the bit where the patterns go, go to hue and saturation, we saturate it completely then lighten it to however you want