In bad neighborhoods it usually means the block is gang-controlled and what kinds of drugs the local gang/dealer sells: white for coke/crack, black for heroin, etc.
That usually marks where someone young(ish) died - gunfire, traffic accident.
All this being said, this pretty much applies only to bad neighborhoods, and even then i wouldn't be surprised if some of it was trophy-related.. although i think the people who do trophy stuff like that really would be more of the tom hanks sort of stuff, not the sneakers on wires and the teddybears and stuff in makeshift telephone pole shrines, etc.
The tom hanks photos truly freaked me the hell out. Bad feelz from those. Bad.