White and metallics are not a typical colors used in home and office printers. ALPS printers are the only non industrial printers that I know of that have the capacity to print white.
You must set up your artwork to print everything but white, leaving the white area transparent so the white decal paper can show where the white is. This presents a problem when the color is a blend of white and another primary color.
You're better off finding someone who does print with an ALPS or can produce one shot print of one sheet at a time. Most who produce decals don't produce them on site. They ship them out to a printer such as Cartograf, Microscale etc and are required to have a minimum run of 200 sheets to make it worthwhile. Draw Decal is about the only one I know of who can do single sheet production. Drop Greg an email, btw he's a great guy to work with.