Believe it or not, Aprilia have never officially released any colour codes. As far as I am aware, "paint matching" the red you require is the only way to go.
A good body repair shop should be able to do this reasonably easy for you.
Someone over on the AF1 forums compiled a list of 'approximate' paints which were the best match available for various models throughout the range. Not sure how easy it would be to get hold of the paint concerned but it may be worth a try