Leaf “Vein” Coloration
A neighbor/friend of mine (who does not have a computer) has a Hibiscus that has the back side of its leaf veins turn black, then the leaves eventually wilt and fall off. She'd like to know what is wrong with the plant. Lack of fertilizer maybe?
I would inspect the plant carefully for signs of insects or bugs. If anything is found you will need to treat the plant.
Also I would recommend light fertilizer. Hibiscus need sunlight and humidity.
Here is an article that will help you.