I have several lemon and orange trees where the leaf is dark green around the veins, but rest of the leaf is much lighter.
Leaf Is Dark Around Veins
Certified GKH Gardening Expert
The trees may be suffering from an iron deficiency. This commonly causes this kind of coloration on the leaves and normally happens either when the watered too much and the nutrients get leeched away or if the pH is too high. Test the soil and correct and nutrient and pH issues found when testing. Also, make sure that the soil is not too porous and is letting the water run away too quickly. Here is more info: https://www.gardeningknowhow.com/plant-problems/environmental/leaf-chlorosis-and-iron.htm