I had to do a bit more work, but now I see my way clear.
Behind the scenes, I also tried it the other way, with green radiating out to red. This is how it looks, beside the red glow version.
I want this quilt to retain its blue/green primary colour scheme, but with red/purple highlights. To me, the red glow version achieves this better, not least because the corners will then be darker blue and green. If I go with the green glow, the whole balance shifts towards a red/purple overall impression, as the corners would then also be red/purple. Not what I want. I also think that the red glow version frames the centre better, rather than blends into it. Sometimes you just have to do the work to settle any internal doubts…
Right, back to the sewing room. I have a whole load of setting out to get on with!