I was told this subforum here would have the best odds for some knowledgeable folks to know what's going on.

I am growing Jalapenos and Serrano peppers from seed in a greenhouse in Spain. Up to a certain height I have them in the GH and later (when the sun is not so intense any more, in late fall) I put them on the sunroof under real sun.

In the GH I have two 300W LED grow lights.

"In principle" the peppers are fine and they just recently started to actually produce their first peppers. This is so amazing!

However, even if all the plants are growing under virtually the same conditions, temperatures and lights, SOME of my plants are very pale and yellow.

(On the picture I put up you can barely see it, but rest assured the circled plant in the middle is very pale compared to the others).

SOME of my plants are also those very dark/rust-colored veins (see pictures) which usually appear on yellow leaves.

And SOME of my plants have gotten entirely dark leaves (like in the small image), here I am not sure whether this is "sooty mold" since I just recently discovered I have a whitefly infestation. (I am already fighting the infestation with Neem oil, insecticidal soap etc.)

The interesting thing is that also those plants I put up on the sunroof are not getting greener, they stay yellow.

I am feedings organic only (actually bought a special organic hot pepper fertilizer) and I am also using CalMag (calcium/magnesium). But not matter what, the yellow ones won't become green. You will also notice that the yellow plant I circled, the leaves are more "crumpled" than the others and they're bending inwards and upwards on the side too. Most of my plants (even the green ones) show some "crumpling" but not as strong as the yellow one and none is bending up on the sides I think. The baffling thing is that some of the plants look awesome and grow like crazy...while the next one over might have all dark leaves...or the entire plant has yellow leaves. I also want to mention that we went through several months of extreme heat, and in June/July pretty much all growth has halted due to extreme temperatures...so I wonder whether this has something to do with it.

Edit: Took the picture in the greenhouse late afternoon under PINK grow lights, so I had to manually color-correct the picture. You would likely better see that the circled plant looks really yellow/pale if I had taken them out and made pictures in day-light. But you get the idea hopefully.

Thanks!