Is it legal to discriminate during the hiring process based on hair color?

No. It is also illegal to discriminate against age, sex, and religion when hiring. An employer can not ask you how old you are, if you are married/unmarried and to whom, what religion you are. They can only ask about experience and education.

Hold on a minute...

Discriminate against age, sex, religion, etc... is protected under law. However, that is completely different from the issue of an applicant's hair color. Hair color and other seemly unprofessional looks fall under what most companies describe as "dress code". It is part of a company manual and policy. Most companies have it written in their "Dress code" what constitutes a professional appearance. The dress code is a little different from place to place. It depends on the line of work.

I have to say that most companies will have a clause in their "dress code" prohibiting hair color not found in nature (blue or pink for example). Also, they usually mention something about facial piercings and visible tattoos being forbidden at the work place.

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And yes, it is legal. And yes it is discrimination (in my book but not according to the law... YET). History shows that at one point, it was illegal for women to vote, but with enough women to uprise, what was once considered the "standard norm" was changed. Hopefully in time, what is considered "professional" will be less concerned with superficialism, and more focused on work merit. I encourage anyone who is being discriminated against on the basis of hair color, tattoos, piercings, to let the employer know that they are Discriminating against you.

enough people saying that it is time for this socially-accepted form of discrimination to be changed, in time it will no longer be okay or "acceptable discrimination in the name of having a professional dress code". Just because someone has blue hair does not make them a less qualified applicant for a job.