You'll come across an icon of a spray can on the map. You can choose to do the red stencil for Evil Karma or the blue stencil for Good Karma.
To do the stencil, you have to turn the Dualshock sideways, shake it a bit, and then hold R2 down while you paint. It's usually 3 colors, but sometimes it's only 2.
They all show up on the map so it's pretty easy to get them all.