Blue Shampoo, and When to Reach for Purple Instead

In 1704 Isaac Newton published Opticks, and in it he bent the rainbow into a circle. Mix two colors that sit opposite each other on that circle, he wrote, and you do not get a new color so much as "some faint anonymous Colour." Three centuries later, that idea is sitting on the edge of your bathtub. It is the whole reason blue shampoo works.

Here is what happens to brown hair. Colored and lightened brunette shades have warm pigment underneath, and weeks of sun, minerals in the water and hot showers wear away the cooler tones on top. What is left shows through as orange. Blue sits across from orange on the wheel, so a blue shampoo leaves behind just enough cool pigment to pull that warmth back toward a neutral brown. Purple does the same job for yellow, which is why blondes and silver heads reach for it.

So before you buy either, look at your hair in daylight, by a window, not under the bathroom bulb. If the brass reads copper or rust, you want blue. If it reads lemon or straw, you want purple. If you honestly cannot tell, you probably have a little of both, and the answer is a gentle color-safe wash most days with a toning wash once a week.

The first grid is for brunettes and anyone with orange creeping in. The second is for blondes, highlights and gray. Both are full of formulas I would actually keep in my shower.

Here is a test I like. After you wash and dry your hair, hold a white paper towel behind a section near your face, outside if you can. Against pure white, orange shows up in a way it never does in the mirror. Do it again after two weeks of a blue formula and you will know whether it is working.

If you are a brunette, especially one with highlights or balayage, and your cool brown keeps drifting toward copper, blue is your color. True blue formulas are rarer than purple ones at the big counters, and Virtue's ColorKick is the one I would start with. It is sulfate-free, tones with natural colorants, and works on natural hair as well as color-treated. Its partner, the ColorKick Color-Illuminating & Hydrating Mask, is the brass-fighting treatment for the weeks between appointments, with UV protection built in. Moroccanoil's Color Depositing Mask works differently: it lays down temporary color while it conditions, which is handy for refreshing a shade that has gone flat.

The rest of this grid is prevention, the everyday color care that slows the fade in the first place. Kérastase's Chroma Absolu shampoo is sulfate-free and made for fine to medium color-treated hair that needs moisture without weight. Davines MINU uses green coffee extract to slow fading from frequent washing, and Oribe's Beautiful Color Shampoo is the richly moisturizing one, made to protect hair from the elements. Moroccanoil's Dry Shampoo Dark Tones absorbs oil without the chalky residue ordinary dry shampoo leaves on brown hair.

Adwoa Beauty's Blue Tansy Clarifying Gel Shampoo is here for buildup. Use it now and then to clear the slate so everything else can reach the hair.

What I would skip: toning right before a color appointment. Your colorist needs to see the real brass.

Purple shampoo is the one most people already own, and most people use it too often. It is a treatment, not a daily wash. Once a week is plenty for most blondes, and toning more often than that, or leaving it on too long, can tint very pale or porous ends faintly lilac.

A simple rule for picking one: the paler and more fragile your hair, the more care the formula should bring. Olaplex's No.4P is ultra-concentrated and carries the brand's bond-rebuilding ingredient, so it suits bleached hair that also has breakage. Kérastase's Blond Absolu is an ultra-violet formula with hyaluronic acid to strengthen, best on cool, heavily lightened blonde. Moroccanoil's Blonde Perfecting shampoo is sulfate-free and also made for lightened brunette, which makes it a nice bridge between the two grids.

K18's TripleBright is a foam, and it goes after the cause as well as the color: it removes the metals and minerals that turn blonde brassy, and it is designed not to stain or over-tone. If your water is hard, start there. Oribe's Bright Blonde is a daily shampoo, which is rare in this category, and it is made for silver too. Shu Uemura's Yūbi uses Japanese cool pigment technology and white peony and adds a subtle shine. Dae's Violet Hour and Verb's Brightening Purple are the easy picks, the first with cactus flower and quinoa protein for moisture, the second vegan and made with acai.

What I would skip: purple shampoo on a honey or golden blonde you actually like. It will cool it off.

I think blue shampoo is one of the most useful things a brunette can own, and one of the least known. Most people with orange creeping into their color assume they need a salon toner, when a bottle in the shower can do the upkeep for weeks.

Start with one blue or purple wash a week and look at your hair in daylight before you change anything. Give it until your next trim. If the brass is fading, you have found your rhythm. If it is not, the problem is deeper than toning, and that is a conversation for your colorist. If you want to keep browsing, I also put together Hair Toner at Home and The Best Shampoo for Gray Hair.