2nd week of flower everything is yellow

Ok here are the details. Thanks in advance guys. Trying to get all the kinks worked out.

6x6 Rockwool in a bucket system with hydroton.
Ph it ranging from 5.9 to 6.3 the ppm is around 1200
80 degrees during the day 68 during the night about 48% humidity res temp is around 60
Twice a day feed 1 hour long each time before sun up then right before sun down
they went from veg perfect then slowly started changing over to this.

Everything looks purple to me Pic w/o the blurple?

Two weeks in the plants have stretched significantly and are setting bud. The yellowing on the lower fan leaves indicates a N deficiency. This is normal as the plants burned a bunch of N during stretch. A simple fix is to run half veg half bloom nutrients for a week. A little extra N to replace what was used up.