On citrus and some other sub-tropical plants it is common for older leaves to turn yellow and drop during winter months when the weather is cold. There is less root growth at this time of year, and some nutrients may not be absorbed as well by roots until the weather warms again in spring or early summer. A fertilizer containing nitrogen is usually applied a few weeks before or when new leaf growth begins.
The loss of green color between the veins on new foliage may be caused by a lack of zinc or other micronutrient. If zinc deficiency persists, when the weather remains warm, it may help to spray new, half-grown, leaves with a chelated fertilizer containing zinc, such as "Citrus Grower Blend" by Grow More. A chelated fertilizer or zinc sulfate can also be applied to soil at the edge of a trees drip line.
Iron deficiency may also occur even though most soils contain an adequate amount of iron. Iron may not be in a chemical form which roots can absorb due to high soil pH or poor soil aeration (over-irrigation or poor drainage).