I know that lots of gyarus in particular are very fond of lightening their hair. As are many other girls. But several weeks or so after the hairdresser visit, or after doing it yourself if you’re brave/skilled enough, there is the dreaded yellow orange tone that shows up, totally changing the lovely colour you had achieved. There are many ways to prevent this from happening, but I find the easiest and fastest way is purple.
As you can see on the colour wheel, purple is opposite yellow. So putting purple on top of yellowed hair will neutralise the yellow tone. Getting rid of brassiness.
The way to apply purple to your hair is by using a toner, you can also get shampoos which include a slight purple hint, but I find the toner is much stronger and can be mixed in with shampoo anyway. This makes it a whole lot cheaper. I keep a bottle of toner in the shower next to my shampoo and just apply when its necessary.
The toner is usually applied after shampoo and before conditioner. (Though different brands may differ.) The colour will stay longer if you make the shower colder after toning, locking in the colour. If you over-use the toner or leave it in too long your hair may take a slight purple hint, but this will fade quickly. You can get inexpensive purple toner from places like Amazon.
I would recommend only using it every other wash to prevent lilac hair.