Prescribed, corrective colour contact lenses are actually more popular than lenses that are purely cosmetic though; sometimes people prefer certain tints to help them see their environment better (colorblind people have had mild success with tinted lenses that make it easier for them to discern colors), others like to have tinted lenses that can be found easily when they are dropped on the ground or in the sink, while still others simply want a new eye color.
Contact lenses are classified as corrective, therapeutic, and cosmetic. For those who are short sighted, near sighted, or who have astigmatism (all refractive defects of the eye) corrective contact lenses are in order. In these cases, an eye patient will mostly likely be prescribed spherical lenses. Therapeutic contact lenses cure other problems that are non refractive, like dry eye, and some types of therapeutic lenses even deliver medicine to the eye.
Two types of cosmetic lenses are colour contact lenses and special effects contact lenses. While these lenses occasionally correct vision when needed (they are basically spherical lenses), they also can blur vision because of their design. Colour contact lenses essentially change the appearance of the eye.
Some people may require colour contact lenses for less superficial reasons (perhaps they have a physical deformity on their iris) while others may simply want to ditch their natural brown eye color for a more appealing blue. Natural shades like hazel or blue are available but so are wilder colors like purple. A subtler colour contact lens is the tinted lens, which enhances one's natural eye color, as opposed to drastically changing it.
It is important that a person finds colour contact lenses that suit their natural features though; otherwise their eyes may look strange.