Check out the nail art supplies at Sally beauty supply. Those stores are everywhere. That is pretty durable.
Look up Friendly plastic. These are thin plastic sheets that are heated with a blow dryer to make what ever shape that you desire. People into arts and crafts use this to make jewelry and re-design pen cases. Scroll down this site and you'll see examples. The Art of Friendly Plastic: Friendly Plastic Pen Fever - By Mylene Hillam
This is what the sheets should look like. One and a half inch by seven inches. 6 Friendly Plastic 7" LOT Solid Mix Metallic Sticks - eBay (item 180368174548 end time Jul-12-09 09:21:01 PDT)
Of course, that's for a pack in the example above. But you can go to most arts and crafts store and purchase individual sheets for a buck fifty or online. There are hundreds of colors and patterns to choose from.
Also, if one should go this route, which I haven't done yet, but will for a future box mod. This plastic cannot take over exposure to sunlight, meaning don't leave it on the dash, window seal, or on the heater....which shouldn't happen to your ecigs anyway for the reason of the delicate battery in there. But once that plastic has cooled, it's really durable.
I would say painting the Ecig itself is probably not the best way. Since an ecig is just 3 pieces (battery,Atomizer and Cart) and all of those pieces will go bad. At that point you would have to get another and paint it to match.8-o
Instead how about a tube that you would slide the Ecig into then decorating that.