blue light
Blue neon lights were successfully used in public bathrooms and publicly accessible toilets, as a means of preventing drug users from injecting themselves. Because it makes veins harder to see, it is expected that drug users will stop using these bathrooms for the aforementioned purpose.
further reading:
http://www.theargus.co.uk/archive/1999/02/18/The+Argus+Archive/
http://news.bbc.co.uk/2/hi/uk_news/england/cambridgeshire/3147170.stm