Newer growth is lighter in color and thicker than ...

hey there, that yellowing between the veins looks like the start of a magnesium deficiency , it's probably related to the genetics just needing a bit more as well as the start of flowering needing a bit more magnesium. Nothing easier and quicker to fix, just spray a cal mag solution on before daybreak every ten days until week 3 of flowering. add it to the feed once a week until your flush. the problem should stop spreading and a full recovery within 10 or so days. Sorry you have to add cal mag , but every plant is a little different . Hope this helps ! 🚀