Monochromatic Graffiti Style in Gastown

There's an alley just off Cambie St. in Gastown that has graffiti from one end to the other, and on both sides. I often see photographers using it as a backdrop to a fashion shoot, to give it some edge and street-legitimacy.  As I rode through it one day, all I could smell and see was urine and dumpsters, so I know it's not a "fake alley" with graffiti; it's the real thing. Much of the art is simple and cliche, but I noticed one that stood out. It's a single colour piece, but with different shades.  I wonder if the artist had multiple shades of black and gray spray paint, or if there is some technique to create the different shades? It's a pretty complicated piece of graffiti, working with shades instead of colours, so I thought I would share......