What does purple shampoo do to white hair

When I got blonde highlights, my stylist recommended that I start using purple shampoo to keep my color fresh. This might be a silly question, but wouldn't putting purple on my hair make it look... purple, instead of blonde? She said that the purple is supposed to make my color stay bright, but how does it make sense that putting a darker pigment on my hair would make it brighter instead of duller? Couldn't I just skip all this purple stuff and use a color-safe shampoo instead?

Thanks for writing, Rita! You're totally right - Purple shampoo is a pretty unintuitive concept. When you first think about it, it feels illogical to put purple pigments on blonde hair to keep it blonde. To answer your questions, let's first take a look at the purpose of purple shampoo, and how it works.

Purple shampoo is a type of shampoo that was formulated to contain teeny-tiny molecules of purple dye. When purple dye (in particular, Ext Violet #2),  interacts with a yellow-y head of blonde hair, it makes the hair appear less yellow. Think of it as an at-home toning system to help you hide brassy undertones in blonde hair. The effects are temporary, so regular usage of purple shampoo is a must in order to see results.

How does purple shampoo work?

If you're familiar with color theory, you'll know that purple and yellow are complementary colors. This means that they are directly opposite from one another on the color wheel.

When complementary colors are combined in equal proportions, they effectively cancel each other out.

If you mix a pure purple with a pure yellow in completely equal amounts, the result will be neither purple nor yellow, but a dark grey or ashy-brown color. A different effect is created when complementary colors are combined in unequal proportions. If you mix a lot of pure yellow with a little bit of pure purple, you'll get a less intense yellow. Similarly, if you mix a lot of pure purple with a little bit of pure yellow, you'll get a less intense purple.

Using this logic, you can start to understand how purple shampoo works its magic on yellow-blonde hair. When a little purple dye is combined with a lot of yellow hair, the saturation of the yellow is decreased, leaving a more desirable ashy hue as the final result.

Does purple shampoo lighten hair?

Purple shampoo can't actually lighten your hair. However, it can create an illusion of brightness.

Confused? Let's back it up.

Because it does not contain an ingredient that will chemically change the color of your hair, purple shampoo cannot truly lighten hair. If anything, purple shampoo may make your hair temporarily slightly darker.

This is because the inky purple pigments found in purple shampoo are a darker color than the shades of yellow in blonde hair. Even when a very small amount of purple shampoo is applied to blonde hair, the result will likely be at least a little bit darker than before.

Think about it this way; if you've ever drawn on a piece of paper with a yellow colored pencil, the result is pretty light right? Now, If you color over that yellow with a purple pencil, no matter how light the purple may be, the end result will be darker than the yellow you started with. Feel free to test this out for yourself at home to see for yourself.

With all that being said, how is it possible that purple shampoo makes hair appear brighter?

The brightening effect caused by purple shampoo is actually illusionary. It happens because of an effect called bluing. While your hair might be very slightly darker than before, it will not look darker, because of the ways our eyes perceive color.

This is pretty complicated, so let's use an example.

If you've purchased a bleach-free laundry product to whiten white clothes, you're probably more familiar with this phenomenon than you may think. Like grey or blonde hair, white clothing can acquire a yellow color after continuous use. Yellowing causes the crisp, cold white of clothing to become warmer, and appear darker and more used.

By adding trace amounts of fluorescent blue colors to detergents, laundry products are able to re-introduce a cool tone to these fabrics, and make them appear lighter, colder, and less yellow, which translates to looking fresher and newer.

As humans, we have learned to associate certain yellowy warm tones (such as the faded yellow color of old white clothing) on certain surfaces (like hair or fabric) with environmental grime, sweat, and general icky-ness. We generally perceive these colors as being more undesirable and less fresh.

When cooling hues are added to mask these warm tones, the result feels fresher to the eye, and therefore brighter and more desirable. The less warm, the better we think it is.

Who knew that your hair and your laundry had so much in common, right?

What's the difference between purple shampoo and a color safe shampoo?

A color safe shampoo is a product that contains a mild surfactant (the cleansing agent in your shampoo). While aggressive, super foamy surfactants are pretty good at keeping hair squeaky clean, they also are more likely to damage the cuticle of the hair (the outermost layer of the hair). It's important to keep the hair cuticle intact since it is the hair fiber's first line of defense. The hair cuticle is to the hair fiber what skin is to your body - it provides a protective barrier to keep the inside stuff inside, and the outside stuff outside. So when the hair cuticle is damaged, artificially dyed color fades more rapidly - there's no protective barrier to keep it "in the hair", so to speak.

A color-safe shampoo is different from a purple shampoo since they're formulated to accomplish different goals. While a color-safe shampoo is designed to be gentle on the hair cuticle, a purple shampoo adds pigments on top of the hair cuticle to keep the hair color the desired shade. While it certainly won't hurt your hair to use a color-safe shampoo instead of a purple shampoo, the color-safe shampoo won't prevent the yellowing that tends to happen to dyed blonde hair.
Also - most purple shampoos are color-safe shampoos, but not all color shampoos are purple shampoos. Seeing as purple shampoos are, by default, formulated for blonde hair that has been color-treated, most of them are also formulated to include gentle surfactants that put them in the category of color-safe shampoo.

Can purple shampoo dye your hair purple?

That being said, some people do try to tint their hair purple using purple shampoo by deliberately going against the manufacturer's instructions and leaving the purple shampoo in their hair for hours at a time, instead of for only a couple of minutes. While it's not good for the hair fiber to be covered in shampoo for several hours, it may result in hair that's temporarily tinted purple. This effect will be temporary - as soon as the person returns to washing normally (aka having purple shampoo on for only a few minutes), the tint will noticeably fade. 

As long as they follow the manufacturer's instructions, most people are not at risk for accidentally tinting their hair with purple shampoo. If you have hair that is a very light blonde or is already a very cool blonde, you might be at a slightly higher risk for accidentally tinting your hair purple. If you do see a purple tint, wash your hair a couple of times with non-purple shampoo - the tint should be eliminated.

Where can I buy purple shampoo?

As we've established, purple shampoo is a pretty complicated aspect of cosmetic chemistry and should be formulated by the best experts available. Check out our hair quiz, and have a chemist make an affordable purple shampoo just for you.

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