no its not normal many growers tend to believe that yellowing is normal cause plant is ending its life but in reality the goal is to keep it green as long as you can most flowering nutes will have less Nitrogen and this is what most growers tend to become negligent
in flowering you need to maintain a decent nitrogen mix into nutes , as well in flowering you got to hit them hard with NUTES to create big buds for instance i am ending week 3 in flowering and mt PPM is around 1800 plants are green heres my last grow late in flowering and green