Darkening hair can lead to rich, lustrous color…or a disappointing, muddy mess. To avoid the latter, says Hollywood’s mane color manKim Vo, a good rule of thumb for formulations is that if you’re ever questioning how deep or dark to take the hair, always aim lighter—about one full level above your aimed target. Adding some extra warmth to your formula doesn’t hurt either. And, to prevent hair from appearing dull or ashy when darkening, you might need to apply another color or glaze after the initial color is processed. Prevent premature color fading and add life to your color by glazing with a demi-permanent formula.