Well, I can see the paint stains and dripping from behind the design, and the way you painted it looks almost as if you had created a transparency in Photoshop and pasted it. It's frigging awesome the way you painted it. I'm not very experienced with spray cans yet, so I'm asking if it's a weaker spray of paint that introduces the color, but stays faint enough to see the background of the wall.
A very good technique for me to master, as I would like to create a bold background, then cover with a design like this for a new effect.
mh, well for the drop shadow I just increased the distance and faded with a dark colour, but you also can use some "transparent" colours, witch contain more clear varnish so it doesn't cover that well. Molotow and the MTN Aliens got a transparent color range. You might check this one out: [link]