Three capfuls of Rit Dark Brown with 4ish cups of water.
I get OD green. Not quite the same green as at Marv's using his water and equipment. His is more of a forest green.
I think we've confirmed that most of Rit's browns react with Magpul's sand to make a green instead of a brown.
Since yellow (which works normally) and purple (which works normally) make brown, I wonder if those combined would give a brown on the magazine.
It doesn't contrast too badly with foliage green...