The main reason behind this problem white hair is genetic or hereditary history, the follicles in the hair, lack black pigmentation and they start getting gray and then becomes white at times.
You can stop this by only using lemon and coconut oil. You need to make a mixture of 1/4 cup coconut oil with 1 lemon juice. then you have apply it on your scalp for at lat 3 hours. For best getting best result, leave it for over night and then wash with a mild shampoo. Do it at last twice a week and your white hair will be black again.