Right, that's the brown that he doesn't want.
I'd prime 'em black, and start with a basecoat of a dark hunter green. If you thin foundation paints enough, they're usable, so Orkhide Shade would work. From there, I'd just slowly add white to build layers. When you get to a sort of sickly green-white, I'd wash it with a 50/50 mix of green and chestnut ink that you've watered down SIGNIFICANTLY. You want the slight shine to indicate the slimy look, but you don't want to completely stain the armour, either. Watering the ink down a lot will focus it mostly in the crevasses, which is really where you're aiming it towards anyway. See how that works out.
If you have an old miniature of any sort lying around, you can use it as a test piece. It doesn't have to be a Space Marine model to work out the technique with, after all. You can adjust the colors as you see fit on a test model, and work a bit with the mixes until they're to your liking.
