Gray hair keeps going brassy

I was telling a co-worker about all the awesome stuff I've learned on this board. So she asked me how to get gray hair to dye fully. Her mother keeps trying to dye her gray hair and she said it keeps going brassy. Aparently she has natural medium-light brown hair & keeps buying kits at walmart to cover them. I told her that was one of her problems, but would come back to find out if there was a direction I could send her :).

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Another thing I read recently about grey hair...if you're dying it darker than grey, with some people it just doesn't stay to the true color for very long--it fades, turns brassy, etc. They say those with grey should go at least a shade or two lighter. Now, that would work with me because I'm naturally dark blond (light brown), but those that have naturally dark hair, I have no idea.