Yes, actually. If you are living in the United States, you will need to get an actual prescription before you can buy purple contact lenses, whether or not they are going to have vision correction powers or not.
There was a time period where the U.S. government classified colored contact lenses (purple included) as cosmetic enhancements, instead of medical devices.
Eventually the FDA issued an alert aimed at consumers that said it considered the lenses, purple contact lenses included, to prevent risks of possible blindness or other types of injuries to the eye if they were issued without the knowledge and understanding of a professional.
In 2005, it was signed into public law (109-96) [You can view the PDF of the Law here] which can be referred to as the “Plano Contact Lens Law” that all contact lenses, regardless of prescription or vision correction powers to be medical devices which effectly made it required to get an actual prescription or any and all contact lenses, which of course includes purple contacts.
This means that if you are wearing them, you need a valid prescription that was written and assigned by a valid eye doctor and cannot be resold without the proper credentials.
If you do see that purple contacts are being sold somewhere where these requirements are not being met, they will usually be cheaper, possible a lot cheaper than you might have seen before, but keep in mind these vendors are most likely breaking the law and might be charged for doing so.
There are many types of places that might be found selling them; places like convenient stores, novelty shops or beauty salons or parlors.