As it turns out, a number of individuals are experiencing darker brows (and lashes). Our elastin and keratin proteins not only fortify the cells that produce new growth, they activate the body's natural hair-coloring cells. If your healthy hair is predisposed to being dark, that is what you are seeing as the replacement comes in. Plus, the more hair you have (thicker), the darker it will look.
If darker brows are not something that you find desirable, we would recommend to use less serum or back off to 3x per week.