Yellowing of Leaves at the very top of the plant closest to the lights?

Something strange I've never seen before. I have 4 plants 9 days into flowering and I've noticed that the very top leaves closest to the lamp are losing their green. The rest of the planet is completely healthy, nice green vegetation and the plants are growing about an inch to two inches a day. Phenomenal growth requiring me to raise the lights every day or so. But the leaves at the very top of the plants nearest the light are curiously turning yellow. Not nutrient deficiency yellow, but its like the lights are bleaching the plants. Holding a thermometer at plant height shows a temperature of 78 degrees F.

The lamp is a 1000 watt HPS but in order to save electricity I keep it on 750w setting until the last few weeks of flowering. Is it just too much light for these specific sets of leaves? No burning or drying or crinkling of the leaves. They are completely supple, moist and otherwise happy except for the odd loss of pigmentation. Close inspection shows the leaves are green along the veins still though. Odd.

I cant see the pics
If it is bleaching raise the lights.
Imo the leaf needs to be green to work correctly. Some shit about chlorophyll or something.

My phone is broken right now and can't take pics. I did raise the lamps but the things are stretching so fast I check them in the morning and when I come back 10 hours later after work they've gained at least a couple centimeters in height and sometimes more. These monsters doubled in size in 10 days. I love a nice vigorous strain.

Off hand the health of the plant isnt quite enough to deal with the light/transpiration in that upper top part of the plant where hps can push to maximun growth possible forcing you to increase distance and incur a little more stretch and healthier tops/buds.

All being perfect i see this sometimes when one plant is a touch off the others, achieving perfection indoors is pretty hard and right up against a hps can demand it.

I increase my area a little with hps so the distance is a little more plus tent volume of air, slow and steady wins the race.

Off course mere speculation, i am only guessing.