I cannot copy the background colour from one slide to the others

You can simply copy and paste the slide with the background you want using Cmd + C and Cmd + V, then add your content on top.

Another possible way is to use slide layouts. Go to the menu bar and select View > Edit Slide Layouts. This works like a master slide. If you set the background in the layout, any new slides based on that layout will automatically have the same background.