I an not an attorney. Any advice I give is just from experience.
You probably have the right, as you are a small "company." However, do you want a humiliated, irritated employee? Have clients commented?
If she were the dental assistant, I could understand no black nail polish, extensions, diamond chips in nails, etc.
Click on my profile to see what I do. Where I work, the women dress in suits; the men look like they're going out to mow the lawn (see, that's my opinion of their appearance!)
In any case, good luck on your decision.
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I *don't* have an attitude, even if my writing makes me look like I do