I would have thought beige with a lot of grey dust and the odd grass stain. But there are doubtless different flavors. If the existing donkey samples are a little too grey, a mottled fawn would be perhaps more useful.
I think it's just marketing. If you're selling a city suit, you say "wool." If you're trying to get interest in a crunchy organic tweed jacket or fisherman's sweater that smells like old cheese, you say "sheep."