First, and as an initial base of to make a choice from, your contact lenses must address the problem which forces you to use glasses or contact lenses to begin with. Your contact lenses must provide you with better sight by repairing your near sightedness (Myopia), far sightedness (Hyperopia), Astigmatism, or some combination of these sight problems.
Second, the lens must match the eye it is used for. In order to do so, contact lenses come in a variety of combinations of diameter and curvatures. And of course, not all contact lens products are available in every size, or the size you require, and so it is important to ensure that you are purchasing a contact lenses that will fit you before you finalize the purchase.
Third, there may be some additional medical need which limits the selection of the contact lenses. For example, a certain type of contact lenses which matches eyes which tend to dry out frequently.
Having addressed all of the concerns and considerations dictating and limiting your choice, you can now address yourselves to more pleasurable considerations, such as selecting the exact hue of color contact lenses that will change the color of your eyes from brown to emerald green, contact lenses with various pattern imprints and so on.
A minor note:
In certain cases, your optical handicaps are not serious, and do not require optical contact lenses. If this should be the case, you can mix and match your colored contact lenses with the glasses you were on the everyday, and enjoy all worlds.
And one more thing in closing:
Even if you do not suffer from eye trouble, and you intend to purchase color contact lenses, in the event that you had never used contact lenses, approach your eye doctor in order to examine the state of your eyes. There are rare medical conditions in which contact lenses cannot be used, so you want to consult with an Ophthalmologist and rule out this possibility before you purchase color contact lenses.