What type of phlox do you have? The tall growing phlox that bloom in late summer may have lower leaves turn yellow from lack of water or poor drainage or lack of fertilizer in the soil. One application of Osmocote fertilizer in the spring is what I use to keep my tall phlox green and blooming. The low growing phlox is sometimes called rock garden phlox and  acts like a groundcover and blooms in the spring. This variety will turn yellow if a sprinkler hits the foliage so that the plants are too wet or if an underground rodent damages the roots. It rarely needs fertilizer and prefers a quick draining rocky soil. Both types of  phlox like full sun. Perhaps if you sent an image I would have better luck diagnosing the problem. Keep growing, Marianne Binetti