Put gashes of fake blood all over your grey and then for your bay use fake blood gashes as well as a Skelton or mummy or witch
For the mare use black chalk and do a spider web patterns all over her
Paint a skeleton for dark bay
Or maybe paint a huge bite mark on your white mare and take fake blood and let it look like something attacked her
Paint your dark bay with a his skeleton on him maybe.
The mare could be a witch