Yellowing splotches between veins!

[DrJohnCanna]

Hey so I'm growing using LED Cobs in veg, and after 4 weeks they look like the leaves in between the veins have turned a brighter green almost yellow, but the rest is fine.

otherwise they look really healthy!

what do you think it is?

[UrbZ]

Without pics hard to tell..

But I would say nitrogen or iron deficiency. Thats how they exhibit themselves.

[Waltermelon]

Or cal-mag... But without pics.... It didn't happen.