At this time of year it could be the normal colour change as the plant prepares for autumn.
In the mid summer I would have said mineral deficiency.
see what others suggest.

I would say that it's a little early for your rose to start closing down in your part of the country. Nevertheless, I would leave it alone and give it a feed of Toprose or better still Vitax Q4 in April next year.

Thank you both for your replies. Will give it a feed in spring.