A red that was supposed to be classic ends up clownish instead. Darkened hair turns out muddy or green. Over-highlighted strands eclipse the hair’s base color. Prevent these and other haircolor disasters before they start with tips from some top tinters.

1. Create Separate Colors for Separate Zones
Not all areas of the hair are created equal. To get the uniform color your clients want, you often need to create separate formulas for the roots, midshaft and ends. "Don't overlap and pull color through every time a client comes in," says Linda Yodice, artistic director of Paul…