Yes, by all means use the white decale sheets.
Printers do not print " white".
The other possiblity is buying red and black decal sheets and cutting the pieces you need by hand and then applying them to an area that has been given a white base coat of paint.
http://store.yahoo.com/beldecal/moddecpap.html
http://www.micromark.com/
Search "decal paper" at micromark.
And don't forget the "fixative" if you are using inkjet printers.
And they sell a starter kit with different papers and fixative if required.
http://www.tangopapadecals.com/