I want to build an all-green M1A1 Abrams, what color to use if I choose Tamiya or Lifecolor acrylics.

FS-34079, available from Testors Model Master. I don't know of a match for other brands.
Tanks delivered in the 1980's were supposed to be painted in the four-color MERDC camouflage schemes, but most units found this a burden, and left their equipment in the base color (there were twelve colors available in this system, and schemes were devised for every terrain imaginable, but most units lacked the interest and manpower to repaint their equipment every time they traveled somewhere).
Shortly after the M1A1 was introduced, the current three-color NATO scheme was introduced.

thanks for the info Gerald, I cant find the testors MM in local shops, maybe I'll go with Tamiya Nato green.