Nutes are too strong at this stage, the plant doesnt need a whole lot of nutes at this point. Back it down untill the plant shows the signs that you need to start increasing the food, let the plant be the judge of what it needs.
Any new growth will be lighter couloured than any older parts of the plant.Too be honest i would not worry so much about this it will darken has the leaves get older.But i would flush the medium with just water set at 5.5 and leave to dry also like the other guy says i would only be feeding harf strenth feed for now till the plants get a little older.