I suggest vinyl...
That's what I use, some 8-10 year outdoor vinyl. It's what they use for police decals, and basically any other lettering/graphics. The edges are crisp and you can get just about any color under the rainbow. It's easy to apply and once it is there it's staying until you decide to take it off. You can layer colors to get various effects etc.You can get it at most local sign shops or if you are interested I can help you out with it. I make & sell vinyl. I'm not trying to plug my own stuff but there is some good info on my sites about vinyl. I suggest shopping around and see what's out there.www.reflective-studios.com or www.nativstix.com