You may need to add iron. We use it about 3x a year here. I don't know what local brands you can find but I buy Kerex Sequestrine 138. Sometimes I throw a handful of magnesium sulfate (Epsom Salts) around the bush which encourages basal breaks.

At certain times of the year, though, roses naturally lose their leaves due to temps or aging. Our bushes tend to drop leaves in August when the soil temps are at their highest and nutrients either run rampant (like nitrogen) or aren't active. By mid-January, some start dropping leaves again while they're dormant.

It's normal for the leaves to lighten as the days grow shorter and the angle of the sun is less. At this angle much of the light bounces off the earth so is not available to the plants.

I agree with moroseaz even though my location is in the east - the minute it got a lot cooler I started noticing a distinctive yellowing - I think it's when the veins are green but the leaf yellows. Sure enough, research on the Internet indicated that in heavy clay soils like mine roots can have a problem with iron uptake, and suggested adding iron chelate (of course it being the end of the season you can't find any in the stores). I used to add iron chelate each spring, guess I will have to go back to it. It doesn't seem to be lethal but for me tells me I need to do some amending of the soil - wasn't a problem when the temps were warmer, and it was only some of my plants - Abe Lincoln for one.

I'm generally leery of diagnosing plants beyond the basics, but in this case my leaves were very distinctive.