Just keep in mind when designing your decals that you can't print white without a special printer. My understanding is that most modern inkjets will do color decals quite well, though.
Yes, you have to use white decal paper, or pre-paint the surface with the background white and use a clear instead.
The problem with inkjets, on clear is that colours don't tend to stand out no matter how much ink you lay down. If it's on a white/chrome/silver background the colours will show but if you print out a British roundel on clear decal film and place it on a camouflage Spitfire wing, the colours won't pop and will be quite subdued not to mention you won't have white, unless you pre-paint the surface.
Also, let's say you want to do yellow lettering on a clear decal. Again, if you put it on a white painted surface it will look yellow, but over a dark surface it will almost fade away.
There are printers out there that can print white. I'm not talking about ghostwhite on lasers, but UV flatbed printers, but unfortunately unless you have 80K to throw down, you'll have to find a print shop with a UV flatbed that prints CMYKW. Another problem is, that since these printers are so expensive and require a lot of daily maintenance to run, not to mention the inks being expensive...its going to cost you a lot of money to even run a small job since these printers are usually printing out 4x8 foot sheets. Some print shops won't even run a small job since it isn't worth their time.
If you absolutely need white or yellow lettering on a dark background, you could try and emulate the colour and do several test sheets on the decal paper to test out if it will look just like the painted background, but this will take up lots of time and materials. I've tried this. I found almost the perfect olive drab on bond paper, but when I printed it out on decal paper, the colour changed because the media properties were different, like reflectivity, ink absorption and texture.
For example below, the black lines were printed on clear adhesive, and the Moonbase Flag on White adhesive vinyl. The adhesive vinyl and film are thicker than decal film. But if you do inkjet on regular decal film, you have to seal the surface with clear paint so that the colours don't immediately run when you soak it in water to remove the backing.
TazMan2000
