Light-colored top growth could also be Zinc, sulfur or iron deficiency. Probally som others too but I'm pretty sure sulfer and iron deff is rare. That is a dark pheno, so maybe its just a lot of contrast from the baby leaves. It is true that if you are overfeeding you could cause zinc lockout, and the darker leaves could be an indication of being overfed. overall looking healthy,just keep and eye on them.