Editing shipped materials

I am trying to edit one of the standard SU materials to change its colour. It is called Concrete Masonry Unit and is a cross hatched pattern in pale grey. I would like to change the pale grey to some other colour.

If I try to edit the colour in the SU palette, it is the background colour that changes. And if I then apply that, I just get a colour fill, even though the icon suggests it should be overlaid with the crosshatch pattern. Like this:

Then I tried to use the external Image Editor. I have previously set up Gimp as my editor but when I try to invoke it from the icon in the palette, nothing happens.

Anyone know what I am doing wrong?

Make a copy of the material first, as I don’t believe you can or should change the default libraries. Use gimp to make it from scratch actually if you want full control.

As Ben indicates, you can only edit In Model materials. And make sure Gimp is still set up to be the default editor in Preferences.

Even if you delete ll built in models, the reappear on re-launch.

What are you referring to? Your profile indicates you are using Sketchup Free (web) which has no facility for editing materials.

The built-in modules, in the “ShippedMaterials” folder".
I’m using a trial.

Do you mean you are using the trial version of SketchUp Pro? If so, please correct your profile since that is not the free web version.

You can edit In Model materials and save them into a new collection. You cannot edit the shipped materials directly, though. Most serious SketchUp users create their own custom materials collections.