What are contacts called that serve no purpose other than to just have a different eye color?

Cosmetic lenses: Colored contacts or cosmetic contact lenses are what you're referring to.

Color contact lenses: Color contact lenses can be opaque to fully change the appearance of your iris color or tinted to enhance the color of people with lightly colored eyes.. These are made by a number of contact lens companies. It is important never to share or swap contact lenses with anyone else, as these are medical devices.

Cosmetic or theater: Specialized contact lenses are used as special effects in theatrical or other settings, as well as to cosmetically change eye color. It is very important to see your eye care professional to be fitted with any contact lens, even for cosmetic purposes.