well what u are seeing is normal.the semi lighter green ares are were new growth will appear and then strech to a new set of leaves.buttt i still suggest u test your ph. i have 9 mothers growing right now from seed that started with the same variation in the top set of leaves,over 2 days they have sufferd from ph creep and have started to lock out potasium.in that 2 days in a top feed drip.all the top leaves have turned grey,and the leaves are starting to twist.this is all with one one millionth of a degree in ph.