Yes. Non-natural hair color is not a protected characteristic. Even so, there are a few things you may want to consider before making a policy about this:

• “Extreme” hair colors are becoming more and more common, so restricting colors may dampen employee morale or limit your applicant pool.
• You may need to provide exceptions for any coloring that is due to a religious practice or common for a particular ethnicity.
• If you do make a policy, it may be hard to draw lines between what is and is not allowed (e.g., vibrant auburn v. rainbow red). You’ll want to be prepared for that, and make sure managers understand the policy so it can be implemented consistently.

While employers generally have the right to set dress codes and guidelines for professional appearance, it’s best to have a sound business reason for any restrictions. If neon pink hair just feels unprofessional to you but is unlikely to affect your bottom line, it may be best to just let employees express themselves in this way.