Hi, Stuburt -
I'm by no means expert at blending my own colors, can't say how perfectly accurate my mix was. For US Helo Drab I used Tamiya XF-51, (Khaki Drab,) mixed with some Tamiya flat white. I kept adding small amounts of white, until it looked like Model Master Helo Drab. Seemed a good match, looked a bit like an aircraft paint finish that had weathered and faded a bit.
Patrick