Lately, I've noticed an increasing amount of Google searches for a "Spray Paint effect in Photoshop". (Not too long ago, I posted one way to create the effect using Blending modes.) Due to the popularity of the search requests I am going to show you another way to do it using a Brush Preset you'll find in Photoshop CS2,
Select the Paintbrush tool.
Click the Paintbrush Presets in the tool option bar to reveal a Preset for the brush named "Red Spray Paint"