I'm not sure I understand exactly what you mean but I try to help :
1/ If I want a decal to appear correctly on a colored background / pattern I duplicate it to make a shadow (black, white or whatever suits the situation) placing the shadow a little bit on the bottom right for example. You can also duplicate the shadow to put it on the upper left if needed.
2/ The only way (or easiest at least) to have a decal the exact same color as the paint is to use the classic paint (no pearl nor metallic) and brilliant (shiny?) decals. This way, paint and decals will be exactly the same.
I don't know if I answered the question but hopefully it'll help a little bit.