If you are struggling with decals on metallic surfaces (i.e they don’t show up) read below.
Make sure you are using a PBR material (not custom or one of the defaults like plastic or paint) and you have backface colors toggled off in the display properties. This should allow for decals on metal to render correctly.
Ideally this will be fixed soon, and decals will show on most if not all materials properly, but in the meantime this is a workaround that seems to be pretty reliable here.
The decal will inherit the reflection properties of the metal, so for instance currently you cannot put a matte decal on shiny metal, only shiny metal decals on shiny metal, or flat decals on flat metal etc… On the wish list would be the ability to control the reflection properties of the decal, but we are not there yet.
Seeing something different? Please let us know.