I've seen this a few times. It seems to me to be caused by a rainbow effect forming a corona, as it always appears at a roughly fixed angular distance from the sun. The colour of scattered light in clouds is determined by water droplet size and spacing. The higher light spectrum frequencies (violet, blue) are more prone to scattering than the lower end, (red), so the red comes through. Also, reds against a cloud background will be much more noticeable than the other colours, which might well be there, but very difficult to see by comparison..