After a chat with the guys down at GW Cambridge and a little experimentation I've finally hit upon the perfect recipe for the green of my Sons of Horus.
It's a surprisingly simple recipe, using Mechanicum Standard Grey as as base coat, then a liberal wash of Colicia Green. It's created precisely the shade of green I imagined, in a way that's fairly quick and simple to achieve. I actually can't believe how simple it was in the end, considering that I've been experimenting with various combinations of shades and washes for a couple of months now with not the greatest of results.
Once again though, Forgeworld seems oddly attuned to my thoughts. I returned home to find that they'd put up a painting guide for the 4 legions from Betrayal, while I'd been out. Oh well. My way is far simpler and looks far better in my opinion.