EVERYTHING YOU NEED TO KNOW ABOUT PURPLE SHAMPOO FOR BLONDE HAIR
Purple shampoo is an absolute necessity for blondes to use in between hair coloring sessions as it easily helps keeps your blonde beautiful.
Those of us with blonde hair need purple shampoo because of how the hair is lightened in the first place. When a client's hair is lightened, there is an undertone that comes up. To get clients to a richer, more quality blonde, we add a color treatment called a toner, specifically Redken's Shades EQ, to eliminate any undesired undertones, whether that color is yellow or yellow-orange or even a reddish undertone.
Those undertones are cancelled out when the colorist applies a toner, but you may find they start to appear between color services as the toner fades. Insert the need for a purple shampoo to adjust tone at home! 
These undertones can also be caused by things like sun, hard water or heat styling. For this reason, purple shampoo is not just for color-treated blondes. Natural blondes need it, too. In fact, those who’ve been jumping on the unicorn hair trend of purples and blue in their hair can also benefit from purple shampoo to help keep violet hues from fading.​
Keep in mind that purple shampoo does not replace your regular shampooand should only be used once or twice a week. There is such a thing as too much purple. Purple shampoo can be mixed with regular shampoo to add some of the violet pigment into a regular hair cleansing routine without any risk of going overboard.

Redken’s Color Extend Blondage, is formulated with a Triple Acid Protein complex—amino, citric and maleic acids and wheat protein—that neutralizes unwanted undertones and helps strengthen hair’s cuticles while battling brass and keeping blondes bright.  
 
BRASSY HAIR: WHY IT HAPPENS AND HOW TO PREVENT IT
Brassiness refers to the unwanted warm tones that show up in colored hair. It typically happens in dark hair that gets dyed platinum or blonde, but it can also occur in hair that’s been highlighted or in hair that’s been lightened to brown. 
To understand why this occurs, you’ll need to understand what’s happening during the whole hair lightening process. To lift dark hair, bleaching with a mix of ammonia and peroxide is usually a necessary part of the equation. When hair is bleached, melanin, a dark brown to black pigment, gets diluted in a process known as oxidation. 
After bleaching, the next step is usually to apply your haircolor, which deposits artificial pigment onto your locks while removing some of your natural color. 
Brassy haircolor becomes a problem when bleaching or lifting doesn’t get rid of all the underlying pigment in your hair, giving the warm tones an opportunity to reveal themselves. For lightened blonde hair, the underlying pigment is yellow, and for lightened brown to black hair, the underlying pigments are orange to red. When the brassiness starts creeping up, think of it as your natural hair telling you, “Hey, remember us?” 
When brassiness occurs, that also means that among the red, yellow and blue color molecules your hair dye contains, the blue ones have made a faster departure, leaving—you guessed it—just the warm tones. Since the blue color molecules are smaller, they’re broken down easily and fade quicker with every wash. Unfair. 
Fortunately, you have several options when it comes dealing with brassy hair, in terms of both preventing it and fixing it after it happens. 
1. Select a cool haircolor, like one with the word “ash” in the name, since it’s less likely to turn brassy than one that’s warm. If you gravitate toward warmer shades, don’t worry; there are plenty of lighter haircolors you can pull off that have a good balance of cool tones. 
2. Get a toner.Toner, a translucent deposit of haircolor that fades in a few weeks, isn’t just great for altering your haircolor without the commitment. It can be a great brassy hair fix, too. Also known as demi-permanent color, glaze or gloss, toning can correct the unwanted yellow, orange or even red tones lingering on your hair since it contains just enough pigment to improve your haircolor. Plus, the service will enhance the shine levels of your strands in the process, so you’ll be doing your hair multiple favors in just one sitting.  Just remember that a hair gloss yields only temporary results. The good news is you won’t have to deal with that obvious growing out period situation with your roots like you might have to deal with if you decide to color your hair all over again. 
3.Use a purple shampoo. See the article above.
4.Use a shampoo for color-treated hair. Since overusing a color-depositing hair product may end up turning your hair blue or purple, only use it once a week. The rest of the time, apply color-protecting hair products, like Redken Color Extend Magnetics Shampoo, since they’re gentler than regular shampoo and tend to not include sulfate, which can fade haircolor.