Hey Growmie,
This normally only happens when you have nitrogen deficiency or when your ph is off, and this has me leaning more toward there being a problem with your ph. If the PH near your plant's roots is too low or too high, you will see light green leaves starting to emerge. Sometimes, it is difficult for your plant to absorb all the nutrients if its PH level is not quite right. This might result in nutrient deficiency, even if the nutrients are very close to its roots. Flush the plant and when you do check the ph of your run-off and see what that is it's more than likely going to be off.
