Yellowing

What causes Yellowing of new growth?

First thing would be a deficiency or lockout of immobile nutrients (sulfur, iron, most of the micronutrients). The specific presentation can vary depending on which nutrient is lacking, but in general the immobile ones will affect new growth.

And sometimes new growth just comes in a lighter green color, especially during periods of rapid growth like early flower/stretch period, then darkens up as it matures