Employee showed up tattooed and pierced: Can we now implement a dress code?

Q. We are a small “mom and pop” restaurant that promotes a family atmosphere. Recently, one of our waitresses got a tattoo on her forearm and an eyebrow piercing. We do not have a formal dress code, but generally we do not want our employees to display tattoos, and we prefer limiting visible piercings to two in each ear. Because we think the waitress’s appearance is inappropriate for our restaurant, we are considering implementing this policy through a written dress code that we will distribute to all employees. Is our planned dress code legal? ...