Well, you could have painted the letter color (yellow) first. Then added your stick on letters (spelling walker) and hit it with the blue paint. Once the blue paint was dry, pull off the stick on letters.
For this application, and your experience level with painting letters, NUTTSGT's advice is great. It will give you much better results than painting the letters on top of the existing paint. Producing a negative stencil (like bajones238 has done) and painting the actual letters is quite a bit harder to set up and to execute well, in a lot of situations. But it depends on the size and the detail of what you're working on. (He was matching something that was already pretty crudely done, I'm assuming.)
If you've already had the thing powdercoated or painted by a pro, then I'd say the second best option is a vinyl decal and some sort of clear coat.
If you paint it, you'll definitely want to do a couple of practice runs on something else that has a similar texture. You'll see how close to the look you want on the first hit with the paint, and then how much can be cleaned up with a little brush after the first pass dries.