Hair graying is a natural process, you can stop premature hair graying but to avoid them after a certain age is just impossible but this woman turned all her white hair into black without help of any hair color available in market.
What you need to do, is pretty simple
Mix 1 tablespoon of tea powder in 1 cup of water. Make your tea as usual and let it cool down for half an hour. You may add a tablespoon or two of olive oil or coconut oil to help condition your hair.