There are a number of different types of opaque contact lenses, and which you choose will depend on what you want them to do. Some contact lenses are simply tinted so that they are easy to see when you are putting them in your eyes. These will not change your vision at all and it is simply a measure that is made for convenience.
Some people choose to use opaque contact lenses for the fashion element. Many people want to change the color of their eyes, and if they are wearing contact lenses already then it may make sense to try something a little different. For the most part, they will work just like regular contact lenses. Some people may find that the colored part of the lens will sometimes move over the pupil and this can create some vision problems. Obviously, these are temporary and most people won’t be bothered by that too much.
The opaque lenses that are used to change the color of the eye also come in different variations. Some are simply meant to enhance the color of the eye and some are made to change it all together. It is possible for somebody with brown eyes to appear like they have blue eyes. It may be wise to be a little careful with the more drastic changes, as you might want your eyes to look better, but they may actually become too noticeable.
For people who want to change the color of their eyes but don’t actually have a prescription, this is no longer possible. It used to be, but now all contact lenses need a prescription whether they are intended for vision correction or not. It is probably not wise to wear contact lenses for fashion’s sake alone, but for people who are already wearing contact lenses, it is a nice option to have available.