The blue shampoo is supposed to make sure the hair looks silver and not at all brassy. Sometimes even naturally grey hair will still have the faint remnant of the original natural colour, and the last pigment to go is the reddish one.
it's like the old travel poster on the wall that's been there too long and lost some of its colour. Some pigments fade before others, so the poster looks odd. Sometimes the blue shades are what is left; sometimes it's the red. With posters, usually it's the blue shades left because chemically the redder colours are less chemically stable, and so break down faster than the blue.
With hair, it's the other way around.
So if you're in the early stages of grey and also happen to notice the orangey faded look mixed in with your grey, you can use the blue shampoo and it will tone it down a bit.
For hair that is full grey, this helps it look an even more beautiful silver.
With my sister's bleached hair back in the 70s, Magic Silver White was what she used. It came in a small bottle that required you to put a few drops in a jug of warm water, then pour it through your hair. A lot of "blue rinse" ladies used to use it. If you're at all familiar with the Aussie character of Dame Edna (was on Ally McBeal for a while), the inventor of that character has Dame Edna with a characteristic blue hair, a memory from that era. Dame Edna often comes out with lines such as, "It's not blue, it's Wisteria!"
The instructions for the little old lady blue rinse effect was to put in about 10 drops. My sister used about 3 drops. It was just enough to take out the orange effect, but without turning her hair blue.
I use the blue shampoos all the time now. My hair is still orangey, but nowhere near as bad. It looks much more natural these days.
I'm making an appointment to see my hairdresser in the next week or so, I need to talk to her about what to do with my hair for the next (and last, for at least a few years!) wedding. I was trying on dresses today (for the mother of the bride) and my hair looks really washed out, compared to the lovely dresses I tried.
Marg