Closest to white decals

Inkjet printers do not print any light color very well.  No attempt at an off-white will be successful.

The way to do it is to make a blue background area and type, with your graphics program, white letters on that blue background.  The key is matching the blue properly.  Make test prints on glossy paper until you get the color right.  Then, cut out the area containing the lettering.  Just a rectangle around the letters, or whatever shape. You don't need to be that precise if you have a good background color match.

I was searching around and found Decal Pro Fx

http://www.pulsarprofx.com/decalpro/vertical/1_MENU/1b_Overview/Overview.html

Has anyone worked with it before?

You could try printing on a light gray paper, then cut lightly around the outlines--but not through the backing paper.  Then paint the decals white and apply as normal. 

I used this method once, when everyone seemed to think all 8th Army Air Force squadron and plane codes were yellow, and I needed them in a light gray.