Making your own decals

What you do is use white decal paper and print the letters in white and the background in the proper color.   What you will have since the white won't print but the decal paper will show through and the background will be the color of the model.  

If you don't have white decal paper just paint the area you want the white letters/numbers to show through and print the decal on clear decal paper with the proper background color.

You can only print white with special printers, an ALPS for example. Typical inkjet and laser printers don't come with white toners or inks.

The only way to get white is to print on white decal paper, leaving the lettering blank or transparent on your original artwork file. If you try to use white, the printer will try to compensate and fill in the area...a gray as you have discovered.