Coloured Contact Lenses
These lenses change the apparent colour of your eyes. Most standard contact lenses come with a visibility tint - a light blue or green tint added to a lens, to help you see it better during insertion and removal or if you drop it. Since it's a very light tint, it does not affect eye colour.
Companies that make coloured contact lenses have worked hard to mimic the natural iris pattern, by using a multitude of coloured dots on the contact lens that makes it more natural. The centre part of the lens, covering the pupil, is left clear on purpose so that it does not affect how you see.
The pattern and intensity of the dots governs the type of effect of the lenses on your eye. Your natural eye colour can be enhanced or changed. Dramatic effects can be produced, for example, changing brown eyes to blue or green.