This is a limited palette painting. I used Transparent Oxide Red and Cad Yellow Light + black and white for the whole thing except his jeans and checked shirt (a dot of cobalt blue).
It was an exercise to see if I could make the colour punch. At the very end, I added the horse's blaze and a couple of hoof highlights using white, but otherwise I experimented by painting the horse without using any white at all.
To do this, I relied on the underlying un-toned, white, gesso'd board. To achieve the lighter values, I either used (a lot) more Cad Yellow, or I wiped paint off with a cotton bud/Q-tip until the right degree of luminous-ness was achieved, from light bouncing off the white surface underneath the paint.
The background was put in at the end, using a palette knife.