Deficiency in Tree?
Kumquat tree has dark green veins but the rest is pale green to yellow.
Certified GKH Gardening Expert
The tree may be suffering from an iron deficiency. This commonly causes this kind of coloration on the leaves. This normally happens either when the plant is 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, which may let the water run away too quickly.