I'm not sure of the "why" gray hair sometimes yellows. A quick Google seems to give only solutions to the problem.

My DH's grandmother had beautiful white hair. Her hairdresser put some sort of purple rinse on it to keep it white. Her son, my FIL, also had white hair, but with a yellow tinge. It made him look older than he was and sort of haggard. Obviously his mother didn't let him in on the secret of shiny white hair! DH's hair is probably 90% white. We both use Shimmer Lights. I probably don't need to at 30%, but what the heck... My natural color is very dark brown. I like the contrast of the brown and white.

Funny, my gray shows up in the mirror, but not so much in photos.

My role model for going gray gracefully is folksinger Joan Baez.