http://1000footgeneral.blogspot.com/2012/09/dark-ages-color-palettes.html
http://www.modeldads.co.uk/Life-at-the-Front/wordpress/horse-colours-and-patterns/
http://balagan.info/wargamers-guide-to-horse-colours
http://iowagrognard.blogspot.com/2014/01/horse-color-charts.html
http://www.jenniferhoffman.net/horse/horse-color-genetics.html
https://meeples.wordpress.com/2016/08/10/painting-norman-cavalry-the-horses/
My (old) favorite horse color: Rife Butt (Howard Hues) + Tanned Flesh (GW). This combination produces a Chestnut color which varies from a dark brown up to a warm brown/red. Adding Graveyard Bone (Reaper), creates a chalky brown/grey which could be roan or grey.
I think both my Rife Butt and Tanned Flesh have both dried to mud at this point. Ruddy Brown (Reaper) is the base for my most recent unit of cavalry. Also from Reaper, Heartwood Brown for dun, Fair Skin for tails/mains, and Dusky Skin for grey. Foundry triad Chestnut and TerraCotta have joined my painting table. Expect to see them used in the future.
I’ve often tried to answer the question – what would horses of the ancient period look like. Appears my solid chestnuts are all wrong….
An international research team under the direction of the Leibniz Institute for Zoo and Wildlife Research (IZW) in Berlin, Germany is sorting out the millennia by using horse color. The ancients loved spots, they tell us. Spotted and “diluted” horses were more populous from the beginning of domestication until the end of the Roman Empire, whereas solid colors (bay, black and chestnut) were dominant in the Middle Ages. Source: https://equusmagazine.com/blog-equus/coats-many-colors-dna-ancient-horses-54695
A pair of handy reference pages.
Chicago Terrain Factory 