I live in the American south, where the quarter horse is supreme, so the most common color I've seen in sorrel. The rarest seems to be true white. Most people around here don't like whites because they sunburn and are hard to keep clean, as well as wives's tales about white horses being sick all the time/not as strong/not as skilled as other colors.
Here in shetland pony contry ie most equines up here are shetland the most common colour in black anc to go outwith shetland ponies the rarest colour is spotteed as I n stud book rules shetland ponies can't be spotted and as they are the most common breed up here it makes them really rare. And a little bit of topic shetland has now imported their first miniture horse which happens ti ve spotted.