Of course, you have to use white decal paper. The problem is that you have to trim the edges perfectly, else you get a white shadow around the decal. Your decal edges are simple straight lines, so this should not be a problem. I use a a scalple and thin stainess steel ruler to thrim decals.
For trimming complicated designs on white paper, here is what I do. Before starting the decal I scan a sample of the paint. Then, I open the sample and use the color picker tool. I use the result to set the background color. I then open a new image. With most graphics programs you retain the previous pallet when you open a new one. The result is that the result is the color of the paint (or close to it). Now, if you leave 20 or 30 mils trimming error, it will not be that noticible.
Of course, white decals always have a fine white shadow from the edge of the decal. I solve this by dipping the tip of a sharp toothpick in the background color and run it along the edge. Don't put much paint on the pick. You don't want to get paint on the surface of the decal.