When I set an A5 page, the screen shows an A4 page with A5 cropmarks. That’s OK, but when I want a background colour, how do I confine it to the area within the A5 crop marks?
How do you set up A5 page?
Are you using Crop mark extension? The point of crop marks is that the background can bleed beyond the final edge of the page so the colour will be at the edge after cropping (there will be variation in placement of the cuts). It would be normal to choose a size closer to A5 when cropping to A5, say B5.
You might be better off just using the default page generated by LibreOffice.
If the Cropmark file has been set as the default in templates you could remove its default setting to get a plain page. Click File > Templates > Manage Templates, in the dialog that opens find the template with the green tick (means default), right click it and select Reset Default. You should now be able to create a plain A5 page and have Background go to the edge of the page.