Why does gray hair turn yellow

I really don't know why, anyone else?  If she doesn't like it yellow she can get a pigment depositing violet shampoo (the violet will counter the yellow), or call around to salons and see if they offer any more permanent violet options.

Does your mother-in-law  smoke or live with a heavy smoker ?  That  can have an effect  ... even different meds can  do it... the hair is the bodies watse baskets thats why they can trace so much  about toxic intake from your hair all residues from anything and everything ends up there and stays a long time... the grey hair shampoos  can help from shampoo to shampoo.....for more permanent solution a salon  could help have them do a light bleach  and foil in three different colors to blend .... that would look nice  and blend her hair in too ... it is hard to get rid of by yourself  good luck  hope I helped a little Tressa 

Grey/Silver/White hair is similar to having hi-lift blondes, in that it is easily stained. Whether she smokes may not matter. Smoking will yellow hair faster, obviously, but basic things from the enviornment will yellow her hair. Car exhaust, smog, the elements. Product build-up. Minerals in your water. The best thing to combat this is to use a purple based shampoo and conditioner, made for hi-level blonds. She can also visit the salon to have her hair toned as is done on most blondes.
If her hair was darker when she was younger, she was probably never noticing stains before, as we do not see stains as easily on dark colors, (aka a white rug versus a dark brown rug.)

Cheers,
AnnSimone

Would over exposure to sun do it? I know I've seen women whose gray hair looked a little yellow. Isn't there a shampoo out there, that  you use when the hair yellows? Blue shampoo, silver shampoo? Can anybody help me out here. Can't remember.

Maybe she's lacking amino acids. Your hair color comes from the amount of grandules of pigmentation you have in your cortex. The more the darker your hair. It's also hereditary.