Sometimes when a painting doesn't work out, it gets to me. Sometimes it just spurs me to push it further. I moved to different paper (425gsm vs 300gsm). I read part of an article about colour shifting. It's a long article, but definitely worth the read. I now think that my main problem is Helio Turquoise, which I had used in all my previous attempts. A gorgeous, tempting colour when you apply it, Helio Turquoise. But I can't put up with that amount of shifting. It might be fine in sketchbooks in small amounts, but I can't depend on it for a larger piece.
So, here is my final attempt. Still doesn't match what I have in my head (although that's quite vague, I have to admit!). The colours have worked out better, but I want a stronger balance between soft edges and hard edges. This is mostly hard edges, and if I wanted that, I might as well paint with a crayon!
Time to move on!