First of all there are two major types of colored contacts - enhancement color lenses and opaque lenses. Enhancement lenses are suitable for people with light eyes only, and are designed to make your own color brighter or give it a slightly different hue. For example: a person with naturally green eyes can wear blue enhancement color lenses and get most beautiful aqua effect.
Opaque lenses are designed for a more dramatic change. Whether you have light or dark eyes, opaque lenses are not transparent, so they completely replace the natural color of your eyes with the color of the lens.(How to Choose the Color That is Best for You)
What type and color of colored lenses to select depends on the effect you want to achieve.
How to Choose the Color That is Best for You