Are colored costume lenses bad for your eyes?

Can be. If fit by a proper eye professional they should do no harm. If bought on the internet or at a costume shop, they might be of improper fit and may even be non-sterile which could cause some difficulty for the eyes.
Can be. Contact lenses should only be prescribed by an ophthalmologist or optometrist (even if your vision is normal without them). You should never just go buy them on their own. They have to be fit properly on your eye and you need to be shown how to care for and clean the lenses in order to prevent infection. These lenses can be safely worn if they are fit properly and cared for properly.