Ask any colorist why blond and highlighted hair gets dull over time, and they'll likely answer with this common culprit: buildup — of hard minerals in your shower water, chlorine, salt water, and styling products. The residue left behind muddies your hue over time. And while that's just one reason why light hair becomes discolored , it's one that's almost impossible to avoid.If you're like me, you start to notice dulling of your color about five weeks after your salon visit — and that's usually a good month or so before the next scheduled appointment. What's a girl to do in this scenario? An in-salon demineralizing treatment is ideal, but if that's not an option, fret not.French hair colorist Christophe Robin 'seponymous line is often touted as one of the best for color-treated hair — and it is — but this spray is the thing that really won my heart. It's called a lotion, but don't let the name fool you: It's basically a clarifying mist that helps to remove buildup with vinegar and brighten hair with chamomile — without the drying and stripping side effects of an actual clarifying shampoo. (Tip: You should not be using clarifying shampoo if you have toner in your hair, which most blondes do, or you risk getting brassy fast.)Here's how it works: Mist your hair with a few spritzes all over. And, well, that's it. Don't you love when solutions are that simple? The formula works on wet or dry hair, but take my advice: Don't go over three sprays on dry hair — a little dries down nicely; a lot leaves hair flat and limp. (I learned this the hard way.) That being said, feel free to go wild on damp hair. I've been using it three or four times a week, and it's restored brightness and shine like whoa. It's no color treatment, but the results are pretty awesome for how little effort you have to put in. Plus, on this schedule, I'm fairly confident the bottle will last forever.To cover my bases, I checked in with a master of blond, hairstylist Kari Hill , to make sure vinegar-loaded products are safe on color-treated hair. She gave the go-ahead — "It's an old trick!" — as long as the formula isn't tinted deep yellow, as this can stain platinum- or baby-blond colors. (This formula's hue is a very faint yellow, but those with platinum locks should check with their colorist.)Too good to be true? Almost. The downside is that the product smells like vinegar until it dries down, which can be off-putting for some. However, if that's the price of a bright, beautiful blond, I'm willing to pay.Brightening Hair Finish Lotion with Fruit Vinegar, $48, available at Sephora