There are a number of color-changing lipsticks on the market from the 1980s MOODMatcher lipsticks to the vegan/cruelty-free Winky Lux Flower Balm. The marketing pitch claims they create a custom shade based on your mood and/or skin pH. Is that actually possible? What kinds of dyes are used? What kinds of dyes react with pH, and are they food-safe? Does skin pH vary enough to create different shades?