Yes, the white component is missing from the colors,
as most ink printing systems use somewhat translucent colors,
and rely on the white base.
If you look at commercial decals,
many have a white layer under the color layer.
White decal paper will fix it.
Because most of your projects are stuff that many of us can only dream of accomplishing.
If it was my scene, I'd have a hot dog shaped compressor in some grimy color,
with no manufacturers logo and think I had done something great.
Pioneers always have to slog through the mire to reach their goal.
We all benefit from your hard work.
Today almost any fairly new inkjet can do good decals.
Just be sure to use INKJET decal stock (it is different from laser decal stock)
as ink from an inkjet printer is absorbed into the media,
whereas the ink from a laser is fused to the media surface.
Dye-sub is somewhere between them, the ink is vaporized and infused into the surface.
Also put a clear coat over the finished decal, as most inkjet inks are water-soluble.
The spray clear coat and the carrier film will embed the ink protecting it.
The big "gotcha" with decal printing is that printing inks are not like paints.
They are designed to blend on the media so they are almost all translucent.
If you are making a decal for use on a dark object,
you will need to use white decal paper or really play with the colors to get it right.
Try printing something in color on colored paper to see what I mean.
Each of the colors (normally Cyan, Magenta, and Yellow) is semi-transparent,
and relies on the white paper base for some of the color and most of the vibrance.
Easiest is to make decals for light-colored backgrounds, then you can just use clear decal media.
Almost all of the standard inkjet profiles are based around having a white background.
If you look at the pull down for paper selection in most printer drivers,
you will see multiple possibilities.
Some of these deal with resolution and drying time
(smoother papers give better detail but take longer to dry)
and some deal with the luminosity of the paper used
(brighter or duller whites).