Certain chemicals that could be used to dye the contacts can reverse the contours of the lenses and make them scratch your eyeball. This would be very painful and could end up leading to your needing surgery. Even food dyes that are okay to use in the dying of clothing and hair can be damaging to your eye because you are introducing an unknown substance to your eye that your body is immediately going to try and reject.

If the contact lenses are a blue or brown color, adding more color or tint to them is going to make them very dark. This is going to make it incredibly hard to see. Your eyes will become strained from attempting to see through the dark color which can cause you other problems later on. Problems like needing reading glasses or having problems seeing in the dark, like night blindness.

If you need more information related to colored contact lenses then you might want to follow the link to find out more.