I'm a big fan of Aveda, and then especially their Blue Malva line.
As a blonde, when it's been too long since last visit to my wonderful hairdresser, the hair can start to turn a bit yellow-ish (or golden - while light brown is my preferred colour).
These products are the exact solution to that problem, but should not be used too often in a row because then the hair starts to turn a little blue
(which may suit some people, I just can't pull that off)
I forgot to mention the last part to my grandmother, who asked for an advice because her curly, normally white hair was turning a bit yellow.
And the next time I met her,
my little, cute grandmother had light-blue hair.