I tend to agree with aquadel, normal for new growths to have a yellowish tint before leaves start synthethising chlorophyl. As long as they get darker as you go along you are fine.
Whats in your medium and what you feeding? Highly unlikely that you have a zinc or iron def at this stage. N def typically starts with older and lower leaves first. Its a mobile element so plant takes from its less productive leaf and moves N to more efficient ones closer to light. Just a question of efficiency, nature is awesome isnt it