The reason you can't find that color is that it's the instructions to mix a color. The first part (xf-3 and xf 5) are the colors, while the second (:2 and :1) is the ratio for the mix. It's two parts xf3 to one part xf-5. you really need a clean, empty jar (or other mixing place) to both mix and store the left over. Wait until AFTER mixing to add any thinner, so that you can get the proper mix.
So, for instance, you'd mix 2ml xf-3 and 1ml xf-5. You end up with 3ml, then add thinner if you're using an AB.
Clear as mud? Good. Anyway, that's what that means. Tamiya likes to give that info without telling what it means.
Michael
We could change the world, but God won't give us the source code!