I now have 4 kryptonite keys on my key ring that I carry around for my family. These are the ones with a black plastic bulb on the key's bow. In order to make my life easier, I would like to paint it for easy identification. What type of paint do I need to use along with techniques or other advise.
I used nail varnish, it sticks well to the plastic and is very tough. It's easier to apply than spray paint, but a bit more fiddly and forms a thicker layer of paint. I also used it to mark the locks. You get bottles in the most absurd colours in a 1-pound shop in the teenage cosmetics section.
Paint (and even nail varnish) will chip off the plastic sooner or later. The important thing is to get the paint into the grooves where it's protected. On my recent series 2 this means the logo on one side, the key number on the other. I've got an older series 2 with a slightly different key body, and the grooves on that aren't as deep (and in the case of the logo they're wider) which wouldn't be as good.
An alternative: buy a pack of coloured cable ties, put one through the hole on each key, with colours to match on the locks themselves.
My adventures with a failed Abus were the result of attempting to avoid having identical keys. When buying new, it's possible to get keyed alike Kryptonite locks (in some countries), but they won't sell a lock to match an existing key.
So long as you cover the metal part of the key (i.e. the part that goes into the lock) with some tape (e.g. painter's tape), painting is easy.
You may want to roughen up the surface of the plastic a bit with some sandpaper. Then, you could use spray paint (there are ones which are marked as specifically good for plastic -- some of the ones which aren't marked this way will peel off), or if you go to a crafts/hardware store store there will be paints specifically marked for plastic use (but crafts store paint is likely be less durable in the long run than the others). Krylon sells suitable paints under their Fusion for plastic line.
Alternatively, you could just put colored pieces of tape (e.g. colored duct tape) on each bulb.