Xlioness, he would indeed look lovely in white, but I don't think it would stay white for very long...
CP, I was thinking blue, he looks quite handsome in navy but I think the sky blue is surprisingly good on him.
Red Gate, my automatic first thought was hunter green but it looks really odd on him, both in real life and on the computer screen. However, orange looks much better than I thought it would and so does a dark burgundy.
Since he is a packhorse, he will mostly be wearing his leather draw halter, which looks like this
(only it's lined in sheepskin everywhere because of his weird sensitive-roan-skin). I want a halter for general hand leading and short-term tying him, though.
Using the scientific method of drawing on a photo of him in paint, I have therefore narrowed it down to tan, burgundy, sky blue, or orange. Or black. Or navy. Hm, I haven't really narrowed it down at all, have I?