Yellowing and light tops , well light tops are totaly normal in flower. On new growth. It's due to then stretching so much so fast in flower. Now if your lower leafs are yellowing and dying , at the bottom , that would be nitrogen deficiency...honestly your plants looks super healthy and perfect to be truthful. Your gonna have some awesome buds there bud. Great job. If your not feeding. Your gonna wanna start because they use so much nitrogen at the beginning of flower you don't want them to start eating themselves yet. Keep everything nice and green unail about 3 weeks from harvest then let the plant just take up plain old water and then leafs going yellow is a good thing. But don't worry about that new growth being light yellow. Yeah it's probably a good thing to add a half strength grow food feeding rt now then move onto bloom food and drop the grow about 3 weeks into flower. You really wanna keep as many leafs as green and healthy as possible rt up to the very end then let the plant use em as food banks.