A lot of folks associate this bright coloring at the top to a Magnesium deficiency. For awhile I thought the same, until I found out it was just a simple growth spurt, especially when we flip to flower. Magnesium deficiency shows up first at the bottom, not at the top. If anything one could say I'm low on sulfur. And I would think so to if I didn't test the soil this last grow, it was high in sulfur. So...just a growth spurt?