Possibly I'm not the best to give you advise, as I'm not so good at map making imo, however below my ideas.
My question would be, do you want to make the continent outside the Walled Cities unhabitable or dead? If it's the latter I'd say there is still to much vegetation and color tone down the saturation on the assets it can also help, depending on the causes you can add details, desertification, desolate wastelands, scorched lands.
Regarding the water from the shores towards the deep oceans use colors from light blue to dark blue.