Just saw this on Facebook but Moose, the artist, of Manchester, England did this clean-up-the-dirty-wall graffiti more than five years ago in San Francisco. Appeals to me more than spray-painting on outdoor walls.
Surprise: Reverse Graffiti was sponsored/funded by green works, a newish product from Clorox. [Note: Large corporation wants to promote a good-earth product, so uses lower case type for name.]
Ingredients seem environmentally-friendly. But it's not available at New Seasons, very p.c. grocery stores where we usually shop. Have to go to Fred Meyers...try it, share with neighbors to see if might be a better household cleaner (for both residents and service staff) and could be suggested for use in our retirement community.
Still ponder repeating my own on outdoor performance, Kneading to Know, from PSU "Street Art" class, 2011.