Thanks again Andy – that seems to have fixed most of it, but still not quite perfect.
Using the example above, when I’m on the “Issue1” page, the top-level nav item (“Issues”) is white and only issue1 in the sub-menu is grey, as I’d hoped for.
However, when I’m on the “issues” page, the sub-items are now green (I think it’s the default primary green), rather than white as expected.
In the customizer/live preview, there’s no change to the colours for the “current page” at all, but that’s an even smaller issue.
Sorry to keep making work for you 🙂