Sour crack auto 2nd go!

new growth starts off a bit yellow like that and then greens up in the next day or two. If this was a N issues, you would notice it on the bottom of the plant as N is a mobile nutrient and the plant will move it from the lower growth up to the top. Some times this can be a sign on Low Fe aka Iron or even S sulphur. Dr earth has iron in it as a whole ingredient fertilizer but it does take time to break down. The grow looks good on fronts, I have a tendency not to use anything over 18/6 on lighting so your 20/4 light schedule is a bit much for me but if it works for you go with it. When the plants are in the dark they rest up and get rejuvenated and I like to give them a solid 6 hrs. Good Luck!