According to their website, the British Connemara Society doesn't list buckskin as a colour. They list palomino and cream, and then dun. My head hurts from the backwardsness.
I would put buckskin roan on the horse's passport as it is correct. As for breed society, for a half-bred gelding I guess it really doesn't matter what he is listed as. It makes me feel backwards to say it though lol.