If the graphic is white lettering on some easy to cut-out, other colored background (not the color of the prop itself), then yes, I would suggest white decal paper. However, if your just talking about white lettering directly on the prop, cutting out the individual letters from white decal paper would be a nightmare.
I have two suggestions. It the lettering is a common font, like Helvetica, you can get Letraset dry transfers and just rub the white letters on. They come in several common fonts and sizes.
However, if it's something more custom, you can get custom decals made. Usually they're printed with ALPS technology (which is no longer made, but the cartridges are still around on eBay and many decal makers still use it). You can print in white, or silver, or gold even. And many other non-metallic colors as well. I got some custom decals made for some 1/6 WWII crates and ammo I was doing. The yellow and white text are 100% opaque and come on a clear carrier sheet, so they can be applied to any surface.
I got them from this place:
http://www.bedlamcreations.com/waterslidedecals/rfq/index.htm
I cost about $40 for one sheet of decals. You have to supply your own artwork. Bedlam is cool because you can do small quantities. I only ordered one sheet. A lot of places will only do large quantities.