This is one of those fairly harmless pranks you can pull on just about anybody you know. I say fairly harmless because their are the people who can make Crayola scissors dangerous. With a little bit of craftiness and a whole lot of bravery, soon your victim will be a not too happy camper. A roommate is obviously the easiest target but just remember, they know where you live.
Also, a few of these photos do not belong to me, but I used generic photos to avoid copyright issues and stuff. I used gimp on many of these pictures, but if you would like to see the original picture, leave a comment and I'll tell you where I got it from.
Step 1: Materials
Like many pranks, the material have a lot of lee way, depending on your tastes. Basically the main ingredients are
your enemies cologne (must be spray on)
Spray paint ( as microman171 said, using a non toxic hair spray paint might be safer)
Cheap pen (Don't ruin your Monte Blanc)
Step 2: Setting Up Everything
This is one of those pranks that can backfire if you don't do it right. Therefore, we may want to take precautions by setting it up right. First, empty the cologne bottle until it's only 1/8 full. You want to keep a little so the the victim doesn't smell paint in his cologne. Also, use a spray bottle that's about half filled. Next, break your pen apart and keep only the tube part. You're ready.
Step 3: Exposure
Expose the raw nozzle of each can. For spray paint cans you may have to remove the directional cap over the nozzle. For cologne, many already are exposed, but, there may be a small directional cap over the nozzle. Use your arm strength to remove it. Eat beets if you must.
Step 4: Connections
Use your tube from your pen to connect the nozzles. It's supposed to be a fairly tight fit, so expect things to get snug in there.
Step 5: Use the Force But Not Too Much
On a flat surface place the cologne down. The spray can and tube should stand up straight and the whole contraption should be perpendicular to the ground. Use the force and press down on the spray can until you feel a slight sinking feeling. That's the nozzles depressing. Let the whole can of paint empty. This is where the prank can backfire the easiest. Basically, you push to hard at the wrong angle, the pen snaps toward you, and you get paint all over your brand new white shirt.
Step 6: Finish It Off
To finish it off, remove the tube and spray paint from the cologne bottle. Wipe off any paint splatter. Use a little bit of the cologne you emptied out and rub it on the outside of the bottle. This will cover up that paint smell a little more. Let the bottle air dry for about 5 minutes and then wipe the whole bottle. That's about it. Place the bottle of cologne back where you stole it from and wait until morning time.
Step 7: Feeling Even More Devious?
I don't have many other tips except, metallic and neon paint is always better, blaming someone else lets you live a few more days, and do not leave the paint can anywhere close to the cologne. For a few days be on your toes for a counterstrike, from the victim, and his friends. Also always remember pranks come best in combos. Don't just stop at just one.