The other obvious solution is to mask an outline of the lettering swatch, paint it white, the apply the lettered segment on clear film over it. The color of the decal area around the lettering can/should/could? be printed the same color as the rest of the 'panel' it goes on.
NOTE: no "white" decal paper I've ever used was opaque enough to hide an underlying 'panel' color... I even considered painting the film, say, white, THEN printing the other colors atop the paint. A thin enough coat to work with was also pretty translucent, and I had to consider what would be the effect on my ink-jet's nozzles. The heat in laser printing also should be considered.
If painting a white (or other desired background color) - some "maintenance/warning/etc.' legends, etc. were red, possibly some were in yellow. The background color 'field' for such decals should be planned for and applied early enough in the overall finishing process that a masked area doesn't cause obvious raised outline edges above the surface.
NB: I have used colored text on clear decal paper over translucent colored silk - a special effect! Color is changed when sunlight shines through silk then decal! Not always a good thing - only do this if you really mean it...