How they can effect your overall look
Colored contacts can enhance your coloring, by adding some 'oomph' to your looks. However, they can also make you look weird or freaky if they are not the right color or if they are totally not within your season.
Here's an extreme example: for the role in "Being John Malkovich", Cameron Diaz wore dark brown contacts and wore a dark frizzy wig. You can see the effects. Dark brown contacts covering her beautiful icy blue eyes "shuts down" her bright, Clear Spring coloring. Not good.
What you usually see are brown eyed women wearing blue contacts. Again, sometimes it can work, but a lot of times it doesn't.
Paris Hilton is one person who has, I guess you'd say, successfully managed to create the blue eyed blonde look when she's normally a brown eyed brunette. Have you seen her in her natural state? Yes, that's really her!
I read somewhere that she spends over $50,000 a year on her custom made contacts alone! Its great that she has the financial resources to keep up the costs of contacts and bleached hair and sprayed on tans. Most people can't do that and probably wouldn't want to spend so much energy trying to change their natural coloring.
I would recommend you only go for the "enhancement tint" contacts. They don't change your eye color, they only enhance them by making them a bit more intense. They are best for light blue, green and hazel eyes.
If you have darker eyes and you really want to change your eye color, you will need what is called "color tint" contacts.However, don't try to go from deep chocolate brown eyes to icy blue. The look is too unnatural and it will be obvious to others.