I have several antique rose varieties (one each) and one bush has had a yellowing problem from the start. The shoots in spring are yellow and the leaves open yellow. It becomes more green during the summer, but has not flowered in three years. All other bushes are doing well. I have good soil, amended annually with compost and sand. The plant gets full sun and regular watering.
Rose Bush Yellowing – Shoots and Opened Leaves
It sounds like you have everything that the rosebush needs, however the regular watering could be part of the problem. If the soils are holding water well it could be getting too much water thus the yellowing that clears up some as the temps get hotter. The hotter temps help take away some of the moisture thus allowing the root system to get more oxygen and better uptake of nutrients. I would use a moisture meter with as long a probe end as you can find, probe around the rosebush in at least three areas before watering. See what it reads, if it says wet or is even mid range hold off on watering until later. If the watering is truly not the problem, then I would check the pH of the soils and see if that is out of wack. If it is it could be causing some valuable nutrients to be locked up in the soils. They are there but cannot be taken up by the rose due to the pH problem. Hope this helps.
Stan the Roseman
Gardening Know How