Docteur Gecko modifies bus shelter ads using light sensitive colors that are only visible at night. His results, his art, are generally dark or subversive comments on the subject.
"Three years ago, Docteur Gecko found a new way for 'hacking' advertising that you find in bus shelters around the world. What's unique about what Gecko does is that his modifications can only be seen during the night when the city lights the shelters. However, during daytime the ad modified seems to be untouched." - Wooster