You can easily demonstrate that the lipstick's color change has nothing to do with the wearer's mood.
Apply the lipstick to some strips of filter paper. Soak one strip in a dish of vinegar, and another strip in a dish of baking soda solution. You should see a color change that resembles the one you get when the lipstick is applied to skin.
The lipstick is made with weak acid pigments that have a conjugate base form with a strikingly different color (that is, with acid/base indicators).
The pigment is a rather complex molecule with electrons that move freely over most of the molecular framework. The pigment has the particular color it has because this cloud of electrons selectively absorbs only certain wavelengths of light. Those wavelengths happen to be light in the visible range, 400 to 700 nm.
When the pigment undergoes an neutralization reaction 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. (For more detail on how why acid-base indicators undergo color changes, see the feature article "Water To Wine").
The pH of your skin is dependent on a number of factors, including such things as diet, stress, physical activity level, and monthly cycle. These factors and the tremendous variety of natural skin colors probably cause some variation in the color from application to application.
Author: Fred Senese email@example.com