This is where I got to in the end.
It's been an interesting exercise for me but I think on balance I'm happy that I met the brief. There is a lot of red in this quilt but for me it works to enhance the blues, greens and yellows rather than demanding to be the centre of attention. It's a useful notion to have up one's sleeve - that red can be used instead of, say, low volume options.
I have also learned that if I stick with the same block for too long I get bored, then I kind of lose the will to sew at all, so if I want to work along these lines again I will need to build in variety, either by making more than one block or by changing scale or some other jiggery-pokery.