The purple cast comes from the UV lamps which illuminate the Norwich Market toilets. The lights were put there to try and stop junkies from being able to find a blue vein to inject into. This attempt at stopping addicts from using the toilets for this purpose was, of course, a failure. Hence the need to place a spike box for discarded syringes in the facility. The pictograms of a razor blade and a safety pin are red herrings to sooth the worried feelings of mothers sending their children into the public conveniences.