The most important factor while choosing the right lens is your skin colour. The eye colour should compliment your complexion. People often make the mistake of buying a colour that looks amazing in it's casing, but clashes horribly with their skin tone.
To make matters worse, there are no samples from the opticians to find out if the lenses suit you. Here are a some things you need to keep in mind:
~ The power of your glasses/lenses. If you already wear glasses or lenses with some power, you would need to tell that to your optician.
~ Eye infections or conditions. You would need to check with your eye doctor before opting for colored lenses.
~ Your budget and period of use. How much are you willing to pay and for how long? Both will determine the type of lenses you buy.
To know more, you can try these links to help you with your choice:
Choosing colours
Make up tips
Also read: Colour power: learn what's your colour