Praise be to Allah.
There are two types of contact lenses:
Ophthalmic contact lenses: those that are used to correct long- or short-sightedness etc. There is nothing wrong with using this kind of contact lenses on the advice of a specialist eye-doctor.
Coloured cosmetic contact lenses: these come under the rulings on adornment. If a woman wears them for her husband, there is nothing wrong with it. If she is wearing them where others will see her too, then it should not cause any fitnah or temptation. There is also the condition that they should not cause any harm, and there should be no element of cheating or deceiving, such as a woman wearing them when she meets the man who is proposing marriage to her. Also, they should not be extravagantly expensive, because Allaah says (interpretation of the meaning): “… and waste not by extravagance…” [al-An’aam 6:141].
And Allaah knows best.