My guess is that grey hair is hard to color.
I'm not sure if it's from a salon or if it's because they color at home, but in any case, once they've finished the coloring process, and they look in the mirror, wouldn't you think these dear old ladies would try to correct it? But they don't!
I've also heard that shampoo can stain grey hair.
Terrible! Maybe a clarifying shampoo would help?
Plus, to add insult to injury, there was (I think there might still be) such a thing as a "blue rinse", used by old ladies to tame the yellowish tint that grey hair seems to get. Except that when used improperly, it will give you blue hair...
I don't have all the answers, just yet, all I know is that I don't want to be a blue-haired old lady walking around oblivious to the fact that my hair looks like Marge Simpson's!
I should have more or less 20 years to find out, I just hope I'll remember the cure when I'll need it. I'll just have to re-read my blog ;)