How do you get the yellow out of gray hair?
There are a number of products avialable from different makers designed to remove yellow from grey hair. One of the most readily available is “Shimmer Lights” by Clairol, which is usually available in most supermarkets or drug stores. If it's really bad, a quick visit to your local beauty supply store will find what you need. There are also anti-yellowing formula shampoos and conditioners available from some of the larger salon chains. If you have a favorite salon, be sure to ask them about the availability of these products.
In some more worse cases, when dealing with 100% gray hair, you can use a high-lift bleaching service to try and strip out the yellowing or a color-toning service using a platinum shade of blonde (basically a color that uses a blue-violet or violet base color) to neutralize the yellow. However, most people prefer the blueing agents in the anti-yellowing shampoos to a haircolor process, since many of the high-lift bleaching and color services can be harsh on the hair.
Good Luck :)
Use bicarbonate of soda with your favourate shampoo