Here's what my opthalmologist has to say about about purple shampoos. As we get older, the lenses in our eyes age and become yellowed, even if we DON'T develop cataracts. So we tend to perceive a lot of things we used to see as white suddenly develop a yellowish cast. And this is particularly true in the case of our white hair. Sometimes it is truly yellowish, but so often it's largely a matter of our perception. As a result purple shampoos were developed to cut down on this yellow appearance. (It's a simply matter of color physics).
What happens often is that we overperceive the amount of yellow so use an excessive amount of the purple or blue color-- either in a shampoo or a ``rinse'' that we put on our gray hair. That's what ``the blue rinse set'' is a pejorative term used to describe older women who overdo the color correction.
As a side note, a notice as a quilter that older women tend to buy a lot more blue fabric than any other color. I think it's a function of the same color-correction related to yellowing lenses. I've also noticed that when people get cataract surgery, they remark about how bright the colors suddenly seem. It's because now they are looking through brand new clear lenses in their eyes instead of the old yellowish ones they've had all their lives.