Coloured contacts may impair vision. Clear contact lenses were compared to tinted lenses with varying space for pupils, known as the optical zone. Even 1 mm of difference in the optical zone showed a change in vision.
Many of the 30 participants said that with the coloured lenses, they had a hard time distinguishing between things of contrast at night in addition to having blurry vision in general.
There is no claim of dangerous repercussions for wearing coloured contacts, but it is important to know that if you are going to be wearing them for more than a few hours on the off occasion like halloween, consult an eye doctor, and do not buy them over-the-counter. It is when people purchase the contacts over-the-counter that they most often experience difficulty with vision.