I think Katze is on to something. My scalp is pretty oily. I tend to prefer more cleansing shampoos at least every other time I wash in part because the excess sebum makes my hair appear darker. I lose my natural lighter blonde highlights that make my hair color dark blonde and not something else. CO or heavy conditioning after shampoo makes my hair take on a monotone color that I am not crazy about, though CO failed as a washing method for me anyway.