I haven't done a flush on mine, but as usual have a couple relevant general tips to add. I'll let SD de-bunk your write-up there, if need be.
If theres residue inside the overflow tank, I dump a handful of gravel in it with some water and cover the holes then shake the bejeepers out of it to help scrape that off. Dump gravel and rinse thoroughly.
I did fix a mess-up where a co-worker mixed green and not green. From that experience, I would say that when you're in the pure water and running stage, that its important to work the throttle some. Like run it gently up to 2,500 RPM and hold for a few seconds and let off abruptly. If you're watching the discharged water, you'll probably see a surge of color after it comes back down to idle. So in my opinion, throwing a few pressure spikes at it while open and flushing helps evacuate the bad stuff. A quart of wrong left in it may be about as close as you can get.