Having travelled part of the A272 recently and found the signs to be a confusing mix of green-signed and white-backed signs, I emailed West Sussex County Council's highways department. They have clarified that the entire A272 currently has primary route status, although there have been changes in the past and there may be changes in future.
However, it isn't clear whether this applied to the sections of the A272 outside of West Sussex. I suspect either end of the road may be non-primary.
I must remember to tag the West Sussex section accordingly. People keep swapping it around as they see various signs in either background colour. Any suggestions on a good tag to indicate authoritative source? I'm currently thinking source:ref=WSCC highways dept, perhaps followed by a link to this diary post.