Hi David:
As both a modeller and restorer I would stongly advise against using modelling paints on your vehicle. They are usually colour matched to a lighter shade to take into account scale; they use incredibly fine pigments to meet scale needs and they are not robust... all this means that they will fade quickly and and disappoint you in a season or two... this is bitter experience speaking.
Jordan's solution is by far the simplist and has the added bonus of putting a protective coating on the markings. Use a matt clearcoat but make sure its compatable with the paint used for your markings.
The issue of flat or glossy paint for markings has never bothered us at the S&P as the painters circa '39-'45 used what they could get!
There is an interesting tale from the spring of '45 about acquiring paint in France or Belgium after the white-washing of winter '44... lacking paint plants that could make to spec, the camouflage lads in the RE/RCE simply mixed oil drums of gloss black and gloss yellow until it looked OD enough and then dumped in shovel fulls of talcum powder to matt it down... so much for spectrum colour matching!
Units were directed through the camo units and were repainted like a car wash... stay in yer truck mate, we'll paint right over the dirt and oil and then you drive away wet!
Mike