As far as I'm concerned, cosmetics companies that use the word "technology" raise a red flag. It is usually a cover up for lack of any innovation at all. But when I saw DuWop - a makeup brand that I generally like (I recently included DuWop in ourFive Best Antiaging Foundations - refer to "mood lipstick technology" in the blurb for its new Private Red lipstick ($22) I was intrigued. After all, DuWop claims (I have no idea if justifiably) to have brought us the world's first lip plumper.

DuWop promises that, within five minutes, Private Red will metamorphosize into the perfect shade of red for you. I was curious to know how this cosmetic chameleon came about. A cursory look at Private Red's ingredients list didn't enlighten me as to how this was achieved, so I took a little diversion down memory lane. It seems that color changing fads have been around for a while Remember "mood rings" in the 70s and stress testers in 80s? They seemed to be reacting to heat, so perhaps Major Red puts a new slant on hot lips.

This turns out not to be true. Nor do mood lipsticks have anything to do with mood. You can perform a simple test at home should the - ahem - mood take you. Dab some Private Red  to strips of filter paper. Soak one strip in a dish of vinegar and the strip in a dish of baking soda solution. The colors will magically change. The lipstick is made with weak acid pigments that have a conjugate base form with a strikingly different color (an acid/base indicator).

There turns out to be quite a bit of science in this. Pigments complex molecules with electrons that move freely. This cloud of electrons selectively absorbs only certain wavelengths of light and that gives the pigment its color. When the pigment undergoes an neutralization reaction - due to your pH balance - on the surface of the skin, there's a sharp change in the structure of the electron cloud. This sharply changes the wavelengths of light that the pigment absorbs, and the color of the lipstick changes.

So now you know. The pigment is reacting to the pH of your skin, something that changes all the time depending on diet, physical activity, where you are in your monthly cycle.....

Anyway, back to DuWop Private Red. There are a couple of other interesting things about it. As well as all the usual colors that's you would expect to see (think red 7 etc), there are some more unusual ones, at least as far as lipsticks go; henna and annatto, a South American seed used to color food. Be warned that annatto is a very common allergan.

Ingredients in Private Red

Ricinus Communis (Castor) Seed Oil, Beeswax (Cera Alba), Ozokerite, Tocopheryl Acetate, Caprylyl Glycol, Hydrogenated Polyisobutene, Polybutene, Fragrance (Parfum), Carthamus Tinctorius (Safflower) Seed Oil, Lawsonia Inermis (Henna) Extract, Bixa Orellana (Annatto) Seed Oil, Hibiscus Rosa Sinensis Extract. May Contain/Puet Contenir (+/-): Mica (CI 77019), Red 21 (CI 45380), Titanium Dioxide (CI 77891), Iron Oxides (CI 7749, CI 77492), Carmine (CI 75470), Bismuth Oxychloride (CI 77163), Red 6 (CI 15850), Red 7 (CI 15850), Yellow 5 (CI 19140), Manganese Violet (CI 77742), Red 27 (CI 45410), Red 28 (CI 45410)