Painting two-tone numbers is not that hard really... The key is to work with thinned down paints. This helps application, keep the insigna very flat and thin, and already gives a bit of weathered look with a touch of base color showing through...
I start with the color that outline the number, but I paint the whole thing rather than just an outline. On Zimmerit, errors are 'allowed'. After all, even if airbrushed on the real thing, there would have been paint splilling under the stencil... Once the paint is dry I then add in the 'inside' color. Since there is a 'line' to follow, it is easier to do this stage than the first one. Glarring mistakes can the be corrected with paint or scrapped away with a cocktail stick. I normally use enamels for this as up to a certain point, it is easy to scrape minute amount of paint with a cocktail stick and correct little mistakes.