Ok, thanks! I just put together a bad example without normals, my bad. So I added normals and now I can certainly see a difference. I also erased red and green channel in detail.jpg to only use roughness. But it’s only visible as a “specular” roughness, see below:
So I should have read up on the metal-roughness technique and I’m probably wrong in that I can use this for what I want. Isn’t there any other more general way of accomplishing “regular” generic multi-texturing, something like glTexEnv with GL_ADD or GL_MODULATE?