I need to make a bit of catachan green, to touch up my tanks. I have most browns, DA/Goblin/Camo greens, and blacks and whites. My first attempt got a Olive drab color, like the cheap plastic tanks with the army men. Too light for me. Adding black didn't work. And DA green made it too rich. Does anybody know how to make this?

Try adding small quantities of colors, one or two drops at a time instead of large doses. Maybe have a try at adding Bleached Bone, it helped me a bit when I was trying to achieve a similar thing myself.

You need a fair bit of yellow iirc.