From Warhammer Community https://www.warhammer-community.com/2018/11/22/diary-of-a-horse-lord-part-3fw-homepage-post-3/

Dark Brown Horse

1. Skin Rhinox Hide
2. Skin Agrax Earthshade
3. Skin Rhinox Hide Hair Abaddon Black
4. Skin Rhinox Hide mixed with Mournfang Brown (1:1) Hair Eshin Grey
5. Skin Mournfang Brown Hair Dawnstone

Grey Horse

1. Skin Dawnstone Hair Karak Stone
2. Agrax Earthshade
3. Skin Dawnstone Hair Karak Stone
4. Skin Dawnstone mixed with Flayed One Flesh (2:1) Hair Ushabti Bone
5. Skin Dawnstone mixed with Flayed One Flesh (1:1) Hair Rhinox Hide mixed with Lahmian Medium (1:3)

Light Brown Horse

1. Skin Mournfang Brown Hair Rhinox Hide
2. Agrax Earthshade
3. Skin Mournfang Brown Hair Rhinox Hide
4. Skin Mournfang Brown mixed with Flayed One Flesh (2:1) Hair Dryad Bark
5. Skin Mournfang Brown mixed with Flayed One Flesh (1:1) Hair Gorthor Brown

Finishing the horse

1. Leather Gorthor Brown Cloth Deathworld Forest Armour Doombull Brown Hooves Steel Legion Drab Teeth Karak Stone Tongue Bugman's Glow Metal Ironbreaker Gold Retributor Armour
2. Agrax Earthshade all over
3. Leather Baneblade Brown Hooves Karak Stone Cloth Nurgling Green Armour Tuskgor Fur Metal and Gold Stormhost Silver

Theory

1. The ‘Theory’ horse example might look a bit… strange, but it’s there to show where the coats of paint are applied at each stage. Simply follow the theory with the colours noted in the guides below – just click the image to see how it was painted!