Because gray hair has lost all its pigment, it's more susceptible to environmental factors and chemicals in hair products that can turn it yellow over time. Certain medications and illnesses can also cause gray hair to turn yellow. Fortunately, there are natural ways to remove the yellow cast from your gray hair, using common ingredients like apple cider vinegar.
Wash your hair with a natural shampoo specially formulated for gray hair. These usually have a bluish or purplish cast that tones down the yellow, as well as protects your hair from becoming discolored again.
Mix 1 tbsp. of apple cider vinegar into 1 gallon of water, and pour it over your hair. Rinse thoroughly, and follow with a conditioner. Use a conditioner that is specially formulated for gray hair, so it doesn't add to your problem.
Boil 2 cups of water, and add betony flowers. Allow the flowers to steep until the water turns purple. The purple rinse neutralizes the yellow tint. For very yellow hair, wait until it turns a deep violet. If you only have a little yellow, wait until it's a slight tint of violet. Allow the liquid to cool completely. Pour the infusion over the hair, and rinse with water. Follow with conditioner.
Boil 2 cups of water, and add fresh or dried hollyhock flowers. Hollyhocks are another blue-violet flower that makes for a good rinse. Steep the flowers until the water has reached its desired color. Again, you want it to be blue, but not so dark that it gives you an intense tint. Pour the infusion over your hair, and rinse with water.