As for yellow hair that is supposed to be grey/silver, use purple shampoo.
Actually - If anyone just wants to "Brighten-up" or make their "Grey"/"salt & Pepper" /or even Brassy (yellow stained ) hair standout a little more . . . Some of the products to use are Clairol "Shimmering Lights", L'Oreal "Serie Expert Silver Shampoo", or there is also Shiny Silver" by "One & Only", and Sally's Beauty Supply Generic "Shimmer Lights".
All I'll say is be very careful when and how you use these products, especially if you do not want BLUE / Purple Hair. FAIR WARNING ! ! !
NOW - Follow these instructions (AND - Best if done in shower):
Wash hair as you normally would w/regular shampoo to remove dirt/grease/buildup, rinse well with very warm/hot water . . . 2nd shampoo w/ Bluing Shampoo, use less shampoo - maybe the size of a nickel/dime (depending on the length of your hair) or less in the palm of your hand and spread in palm of your hand before applying to hair & scalp. Starting at the roots and working your way down to the ends. Massage the shampoo gently into your hair to create a rich lather wash/cover hair with shampoo - let sit on hair for 1-3 minutes - NO MORE - Then rinse well, condition if need be, and dry as you normally would . . . Just Sayin' . . .