It's normal for growing leaves to appear yellow in the center, especially in the morning.
This happens when they expand during the night and the early morning hours before they've had a chance to "green up."
It's especially prevalent during flowering when nights can exceed 12 hours. This is because, during the night, the plant uses up the energy it made during the day, resulting in new growth.
It has also been observed that plants do most of their leaf tissue expansion during the early morning hours, which is why we sometimes notice a drastic change in plant development when we walk the crop in the morning.
Don't worry about yellow centers as long as the plant greens up during the day.
You only need to become concerned if the yellowing stays or expands. This could signal a nutrient deficiency or pH imbalance.