The reason for yellowing is either incorrect PH or overwatering. I am betting on PH. So you need to correct your PH (6.0 -6.5 rec 6.2), but it will take some days to stabilize your flower. Your yellowing should stop and partially correct itself, but not 100%. Remove extremely yellow only after your flower is stabilized, otherwise yellowing will progress on the remaining leaves if your remove them prior of stabilization. Good luck growing.