Hi there! What is the light you are using? Is it really 600W (power draw from the wall) or advertised as "600W" while being max 200? Nevertheless that yellowing don't look like nuteburn but deficinecy of some sort,. My first though was the light is just a little bit too intense for them and therefore the top part starts to show deficiencies but slowly. If the intensity is waay to high they will burn or buds will bleach (turn all white from some parts)