My Hibiscus Leaves Are Turning Yellow And Falling Off.
I have been fighting aphids also
Certified GKH Gardening Expert
Likely, once you control the aphids, you will control the yellowing.
These articles will help you with insect control:
Here is an article that will help you with the general care of Hibiscus: https://www.gardeningknowhow.com/ornamental/flowers/hibiscus/how-to-care-for-hibiscus-plants.htm