When I grow Basil, it is never as dark green as what I can buy in the store. Everything else in the garden does fine.
My Basil Is Never Dark Green
Certified GKH Gardening Expert
Try adding a bit more nitrogen-rich fertilizer and see if this helps with the greening up of the foliage. If not, it may just be the type of basil you have and is no indication of there being anything wrong as long as it appears healthy otherwise.