As anyone who has gone from blonde to brunette (or vice versa) can attest, hair colorists possess a lot of power. It’s hard to pull off an overall look if your signature shade has gone brassy. Colorists are essentially the beauty industry’s version of impressionist painters who spend years mastering brushstrokes and an intricate balance of tones so that your locks are tinged to your liking. And since individual flourishes make each colorist unique, it’s important that you do your homework before your first appointment. Here, four in-demand Hollywood colorists explain how you can find the pro who is right for you and what to ask for once you do.