Costume contacts sold without a prescription are illegal in the U.S. and pose serious eye risks. The American Academy of Ophthalmology warns against them. The CDC cites risks including "eye sores and vision loss."
Cheap Halloween contacts are typically one-size-fits-all, but contacts should be fitted individually. An ill-fitting contact can cause corneal abrasions, corneal ulcers, and infections like keratitis. The FDA found bacterial contamination in 27% of costume contacts in a 2017 report. Severe infections can lead to blindness.
If you want colored contacts legally, you need a prescription. Prices start at $40 and up at retailers like lens.com, Discount Contacts, and Wicked Eyez. You must provide your prescription information before ordering. The Federal Trade Commission requires eye care providers to give you your prescription after a fitting at no extra charge. You can then use that prescription with other vendors.
If you wear prescription colored contacts, follow basic care rules. Wash your hands before handling them. Store them in contact lens solution and keep the lenses and case clean. Do not sleep or swim while wearing them. Do not share them with others.
Watch for signs of infection including redness, swelling, pain, or excessive discharge. Seek care immediately if you suspect an infection, since eyes are more sensitive to infections than other parts of the body.