The cause of the "deficiency" is not iron, it is manganese. The deficiency symptoms between iron and manganese do have somewhat similar symptoms, and can cause some confusion. An iron deficiency symptom shows NEW leaves with green veins on otherwise yellow leaves with NO border green areas around the vein. The deficiency symptoms for manganese are, newest leaves with green veins WITH GREEN BORDERS to the sides of the veins, on otherwise yellowing leaves. Manganese leaf deficiency symptoms are very often transient, and are due to the marginal supply of available manganese in most soils, which thee tree's roots temporarily are unable to extract enough manganese to supply the growing new leaves. The tree almost always takes care of it self as it adjusts to its new location. I've been growing citrus for 30 years and have never needed to intervene in this transit deficiency. What you are seeing is common. If you wish, an application of manganese sulfate, or manganese nitrate will also resolve the situation, but most growers don't bother.
You can begin fertilizing the tree two weeks after planting. A new 1 and 2 year old tree should be fertilized 4 times during the growing season between early spring and September, equally spaced out through this time..