The current implementation has it so that the decal gets applied to the color channel in Rhino Custom material, and to the base color channel in Physically Based material. This means the decal gets the property of the material it is applied to.
I made a video how you can achieve this type of decal with texture mapping
and the logo from the video made metallic on glass:
Naturally you don’t have to emboss your decal texture. You can do without. And you can pick another material type than metal of course.
For Rhino 8 we are contemplating giving the decal separate material properties.