but for real pics are worth a thousand words and you need to give a shit ton more information about what your doing for us to have any clue on how to help
Generally lightening between the veins in most plants as clorosis or a lack of iron which equates to a ph down. n mj case check your ph, sounds like an adjust may be needed . Also pictures really help a lot.Good luck.
I really can't see Promix having an iron deficiency on a seedling, but I'm still a rookie. I would fill out the form & do some more research first, pics would help as others have stated.