just a idea, but they sell that plastic dip spray paint, that you can peel off, spray that the color you want then use a nice sharp pen knife to trim out the letters , and peel rest off, that should leave the inside of letters the color you want and peel off everything else you didn;t want painted!
and if for what ever reason you don't like the letters colored in, you can also peel it off them rather easy
some folks really like the stuff, doing wheels and such and holds up ?
Just a thought!
also, maybe if you wax all area's you didn;t want painted first, it would peel off there easier too?
just clean out letters real good with some alcohol so it does stick where you want it!