Q.Knockout Roses Have Leaves with Areas That Look Discolored
These areas are not on the leaf but in it.
I am not sure what you mean by discolored. However, it could be that the leaves are just turning towards yellow as they are soon to fall off. It could also be a fungus that is working on them. Here are a couple links to articles that you may find useful:
https://www.gardeningknowhow.com/ornamental/flowers/roses/black-spot-roses.htm
https://www.gardeningknowhow.com/ornamental/flowers/roses/rose-bush-diseases.htm
https://www.gardeningknowhow.com/ornamental/flowers/roses/yellow-rose-leaves.htm
https://www.gardeningknowhow.com/ornamental/flowers/roses/leaves-falling-off-roses.htm
These articles will give you some great information.