Painting lines on external Fuel tanks.....

Have you considered using colored automotive pin striping tape?  They come in wide ranges of color and sizes.

Use rubber rings on round or curved surfaces. There have been a few threads on that method. Search the forum.

Here's another tip from your Uncle Jay;

I do my bombs like this, I paint the whole thing the color of the stripe.  Then I cut a piece of masking tape the width of the stripe, stick it in place where i want the stripe, then repaint the whole thing OD or what ever color.  Take off the masking tape and voila, a painted stripe.,

I have found dry transfers are pretty useful for this as they are somewhat flexible.  

If you are set on painting (transfers can be a bit thick) try cutting 2 really thin strips of Tamiya tape for the very edges of your stripe (maybe 1-2mm wide).  Place those carefully, and then use small pieces of tape to do the surrounding areas.  Cutting the tape for the edges thin will let you tape it around the curves a little better.

Groot