I've been mucking about with wet blending the last few days. The short and the long of it is that I'm learning a lot from my mistakes, and that the key thing I need to nail is the consistency of the paint.
The top three bars ended up with the retarder thinning the paint too much, meaning that the colours didn't mix, but rather simply ran together, to the point I lost control of it. Painted over a base coat split into black and red should have muted the line between the two far better than it did.
The middle bar was me messing about.
The bottom three I went too far the other way, with the paint too thick, so that it dried even as I mixed things together, looking blotchy and streaked.
Somewhere between the two extremes I've found so far lies success. I feel I at least know a little more than when I started out, even if there's nothing to really show for it yet. I'll be knocking up a few more tester sprues in the week and seeing if I can nail it.