What can cause this slightly color change?

This is definitely a molybdenum deficiency. Double check ph.. most of the time that’s the issue. But that’s definitely showing on new growth first from your pics. I also feel your heat is high.. anyway to drop it down?

Less is more, I think your feeding them to much and this is a lockout issue. Your diary entry says on week 4 that your runoff is registering as a EC of 1.037 but going in is .5 you EC should not be flexing that much. need to get it under control and these issues should resolve themselves. I'd also try and keep that PH in check it should be 5.8-5.9 for veg and boosted up to 6.2-6.3 in flower for growing in coco coir on hydro nutrients. Calibrate your meter if you have not in a while. and keep at it. Good Luck!

Hello mate, u r using advanced nutrients, check my diaries for dosage

Looks like the start of a PH issue. Maybe pen is out of calibration? It happened to me.

"It’s in coco so are you getting decent runoff with every watering?" Yes, mostly everytime i have a runoff when i water. I give Cal-Mag 2,5ml for 5 liter water. And i guess the Micro has some Mag inside also? (botanicare: Add 1 tsp (5ml) per gallon of water) "And have you flushed at all? " yes, and buffered the coco coir at begin