This is a relatively simple way to paint bobby pins. Sorry about the picture quality.
First, slide the bobby pins onto a piece of paper. If you are going to tape them to create a design, do it now.
Next, coat the bobby pins with nail polish as many times as needed until the color is opaque. If there is tape on the bobby pin, remove it after you place your last coat while the nail polish is still wet. Wait about ten minutes for the nail polish to dry, then slide the bobby pins off the paper. The nail polish will be dry, but it will not have set yet, so be careful, as you could leave thumbprints on it.
Leave the bobby pins somewhere where the nail polish can set for about 24 hours.