Graffiti IS illegal
Graffiti:
"One or more letters, symbols, figures, etching, scratches, inscriptions, stains, or other markings that disfigure or deface a structure or thing, howsoever made or otherwise affixed on the structure or thing."
If it's legal, It is no longer graffiti.
So really, you can go play with your legal walls.
While we have fun doing illegals.