Before I go into how the product works, a little background on the idea. The concept of MoodMatcher's new Moodgloss is by no means new. Custom-color lip products date back at least 70 years. Tangee released an orange-colored lipstick that goes on clear and transforms into "the perfect shade for you" back in 1922, and the trend took off in the '70s and '80s. Avon came out with the Color Magic Lipstick in 1974, and MoodMatcher's original personalized-color lipsticks came on the scene in 1985. Like most things in beauty, the trend comes back around time and time again.Today, there's Dior Addict Lip Glow — a sheer balm meant to "enhance your natural lip color" and formulated with "color-reviver technology." Sephora has the Color Reveal Lip Balm , which is touted for a similar reason. On the drugstore front, there are mood glosses from Nyx and Smashbox . And then, there are the brightly colored ones: blue from Lipstick Queen , spooky black from Givenchy , and pretty much the whole rainbow from MoodMatcher . While the colors and price tags differ, the marketing claims are all essentially the same: a personalized lip color unlike any other.So how do Moodglosses — and the other color-changing lipsticks on the market — work, exactly? Do they really adjust to fit your ever-changing mood? Well, no, not exactly. It's just like a mood ring doesn't really indicate your mood (I know, childhood ideals shattered) — it reacts to your temperature. The Moodgloss, specifically, mostly reacts with your body chemistry and pH level, according to Falen Rauchwerger, the sales coordinator for MoodMatcher.While the Moodgloss doesn't credit a specific ingredient for its color-changing abilities, a bulk of the other products have one main ingredient in common: Red 27, a "red dye which is colorless when dissolved in a waterless base," according to The Beauty Brains , a website run by cosmetic chemists Randy Schueller and Perry Romanowski. "When it comes in contact with moisture, the change in solubility and pH causes the dye to turn bright-pink," the site reads . "The product appears to change with your personal chemistry because the color changes when it comes in contact with moisture, [which] can come from your skin or even just the humidity in the air."The takeaway: These products aren't as personalized as you may think. Kind of like the same lipstick may look different on different complexions, their colors depend on physical factors. And while the Moodgloss may not be the mood ring of yesteryear in squeezable-tube form, it makes for a pretty interesting conversation starter — and it does take the guesswork out of finding a nice pink shade.