The Solutions: If You've Gone Too Dark... Strip away the color by using a clarifying shampoo that's high in detergent for several days. Two weeks later, re-dye with a color two shades lighter than the first, starting at the ends and finishing with roots.
If You've Gone Too Light... Resist the urge to immediately start over with a deeper hue—freshly processed hair will absorb too much pigment, leaving strands darker than you intended. Instead, wait 24 hours, then color just the roots one shade darker. Once rinsed and dried, your whole head of hair will seem to be a more intense hue.
The Pro Advice: Ignore the image on the front of the box. The chart on the side gives a better idea of the actual shade.
Watch the clock. Keep color on for even a few minutes too long and you could end up with a big surprise.
If you're lightening your locks, apply the formula from your ends to your roots. Washed-out roots are a dead giveaway of a bad dye job. And because the heat from your scalp processes peroxide faster, give the rest of your hair a 15-minute head start before tackling the top.