Layer: Material Assignment

Hello All.
I have a file (Rhino7) structured so that empty labelled layers support sub layers of components that have materials applied to them.
How do I remove the default material from the empty layers?

The rendered image is greyed by the default material, and I want the image to show the vivid red.
The original file looks like this:
When imported into the assembly file, it looks like this:
The default material does not show up on the materials tab.
Here is the file:

Hi Hugo -

You can’t - all layers have a material.

It’s not the default material of a parent layer that is influencing the appearance of the objects, it’s the background that is doing that.

Well, just to nitpick: It’s the reflection of the background/enviromnent. Either use a more diffuse background/environment or make the material less reflective.
HTH, Jakob