The truth is purple shampoo is not going to make your hair any lighter or brighter. Purple shampoo is meant to color correct any yellow tones. So if you're blonde and your hair is starting to get a little more yellow overtime as your toner begins to fade, then you can use your purple shampoo. Do not use it every wash or every other wash after you get your hair done, ONLY use the shampoo when you notice your hair starting to turn yellow.
When you overuse purple shampoo, it can cause your hair to look dull in appearance. It can make your hair also appear too ashy, which may not be the original tone you want.
Two purple shampoos I love to use on my blonde clients are the Aluram Purple Shampoo and the Sudzzfx Blonde Ambition Purple Shampoo. These deposit enough pigment to truly correct the yellow tones, but not too much where I'm rushing to rinse it off immediately. Also, they do not dry out the hair at all (which is hard to find for a purple shampoo!)
I hope with this knowledge you now recognize when and if you should use purple shampoo! Put it down, unless you truly need it of course!