Oh man, I'd love to help, but I don't know much at all about hydroponics. So, I'll run through my troubleshooting process and it might help you figure out of think of what the problem is.
-pH test soil runoff
- Check EC
- Check VPD
- Bottom leaves(brain?) Yellowing:
-brain leaf fading
-mobile nutrient deficiency
- nitrogen if so
-some sort of nutrient lockout?
- Slight chlorosis?
-from what, though?
-genetics and natural color, maybe?
-check roots anyway