You'll still have to buy the paint one way or the other, or pay the $10 for a few of these. Figuring out the cost of a can of paint and how many bolts you can spray with 'em, you'll find this solution to be cheaper.
Find yourself some sandpaper, preferably something made for metal. (I used aluminum oxide paper, but even standard paper should work. The "suggested use" will tell you if it'll take more time or not, that's all)
Take the threaded end of the bolt, and chuck it in your drill.
Set the sandpaper on something flat.
"Drill" the sandpaper until you've removed the raised stuff off the bolt head.
Paint.
Done.
Getting a can of the brush-on kind of black Rust-Oleum will allow you to dip the bolt head too, in case you don't want to be spraying your hand black or don't have a glove to wear while you spray it.