Decals printed in only cmyk color toner and therefore WITHOUT white as a standalone or spot color. Note that with transparant decals the final colors are subject to the influences of the color of the substrate and everything in the that is white will remain transparent on the print. This does not count for decals with a white film.
White waterslide decals printed in only cmyk colors are suitable for all colors substrates, where cutting of areas where no white should be provided is no problem (for example for simple shapes or if a white outline is desirable).
Decals printed in only cmyk colors are best suited for white or very light surfaces, or for prints that require the surface to come through the print and influence its color (for example, with a photo on brushed aluminum).