If you've ever colored your hair blonde, you might have already encountered purple shampoo, which can keep your shade of choice — be it bleach or dirty-blonde — from getting dull and brassy. But what does purple shampoo do for hair, and why is it such a game-changer for hair color? Shampoo alone may seem like a no-brainer, but when you're working with color-treated hair in particular, it helps to have some expert intel. With that in mind, here's what makes purple shampoo an MVP for light hair colors.

What Is Purple Shampoo?

"Purple shampoos cleanse blonde hair while depositing a tone onto it," says Alex Brownsell, celebrity colorist and co-founder of Bleach London. In doing so, it can neutralize and cancel out brassiness in hair.

Think back to art class. "In color theory, purple is opposite yellow on the color wheel to yellow — so the purple color in the shampoo balances out any brassy color," she explains. "Acting as a toner, it helps to reduce those yellow and orange hues and keeps your blonde looking fresh."

And while it's best known for use on blonde hair, it can be used on brunette hair, depending on the tone, to counteract orange-hued brassiness, says celebrity colorist Brian O’Connor, co-founder of haircare brand Good Dye Young.

Benefits of Purple Shampoo

Apart from cancelling out brassiness in blonde tones, purple shampoos can add shine to your shade and overall look, says O'Connor.

Plus, it can help stretch out your time between appointments, since a purple shampoo can combat your shade from fading and help prolong your color, Brownsell says. (And these shampoos are almost always color-safe, so you don't have to worry about it incidentally stripping color.)

How To Use Purple Shampoo

Using purple shampoo is fairly straightforward — but there's more to it than lather, rinse, repeat. For starters, O'Connor recommends lightly cleansing hair with just water to remove product buildup that might hinder the purple shampoo from doing its job. Plus, that ensures your hair is thoroughly saturated, which is important; otherwise, "pigment will grab on and you may end up with purple hair," he says.

Next, work the purple shampoo into a lather and massage it into hair before letting it sit; how long you should let it sit depends on what product you're using and its instructions. Then, simply rinse and follow up with either your conditioner of choice or the corresponding toning conditioner.

There's not really a wrong way to use it, but Brownsell says you'll want to avoid overusing purple shampoo, as it can inadvertently make hair look dull and darker. Plus, she says, "it will grab on to the most porous areas of your hair the most, which could result in an uneven blonde." One to two washes a week should suffice; if you plan to wash more often, alternate it with a regular, color-safe shampoo.

What to Look for in Purple Shampoo

The good news: There are plenty of purple shampoos to choose from, so it's easy to find a formula that works best for your hair type and lifestyle. Brownsell recommends the Bleach London Silver to keep the brassiness away, which uses an ashy-purple tone to keep cool blondes on point. We also like the Amika Bust Your Brass Blonde Purple Shampoo or John Frieda Violet Crush Purple Shampoo.

Bottom line: If you're thinking of going blonde or wanting to really amp up your current yellow shade, a good purple shampoo is key for keeping your hair color fresh and radiant.