Some, like Milwaukee, crack down really hard on gang members for this kind of activity  (when they can catch them) and have even resorted to making the property owner victims of such juvenile territorial expressions responsible for cleaning them up in a vain attempt to discourage the practice. 

Other cities have allowed the more creative urban artists to officially present murals which can be enjoyed by the entire community.  Pride of workmanship, the theory goes, will make the participating gangs police their own creations. 

I really don't know if Denver is one of these communities since this artwork might also have come from their arts community or even from nearby college students.   I do know, however, that this, and several other murals near the downtown Convention Center, are much preferrable to the grey concrete and high tension towers which present themselves as the western gateway into Milwaukee when one comes into town on I-94.