Hey buddy welcome to AFN.. silly question time but is it actual yellowing or just light green?? In the pics they all look a little light green which to my totally untrained eye just looks like fresh new growth?? They look great size wise for 12 days so could it just be that they're putting out fresh 'light green' growth fast?? That said one does look noticably lighter. Subbed to see how you go brother. Good luck