Graffiti, completely legal and a way to clean up your city instead of deface it?  Yes it is possible and it is called reverse graffiti.  Reverse graffiti is basically cleaning dirt in a certain shape, symbol or word to form something recognizable.  The main tools used by reverse graffiti artists are stencils and hydro powered washers. Stencils are placed onto the dirty wall, sidewalk, or sign and then power washed.  Everything inside of the stencil is cleaned showing a bright image against the dirty background.  Since this is eco friendly and not defacing any property, it is completely legal.  One company named Symbollix has taken this art and turned it into a business providing guerilla marketing for advertising campaigns.  Check out this video of the Reverse Graffiti Project.