I have seen people use the color tape meant for RC cars, works ok, but is a tad thick in my opinion. You can print onto clear decal film, this works pretty well if you are putting a dark pin stripe on a light colored body, not very well to do the opposite. I have also seen people use a sharpie marker of a paint marker. You can apply a piece of masking tape to help you keep a clean edge.
All of these can work well but each have their special issues.