M81 is a pretty ubiquitous camo pattern, and has been used almost everywhere in the world by every imaginable faction in the past few decades. Even today it is popular with special forces and paramilitary.
I have what I think is a pretty good method of getting consistent results, so I thought I'd share it (and preserve it as a reference fir myself).
As always, I am mixing colors to get the tones I want.
I think the key to getting the correct look is to always keep the colors cool by adding blue paint. The green should tend towards the blue/gray end of the spectrum. The tan should be just slightly blue as well. Finally, the brown should be closer to purple.
Most modelers start with a green basecoat for M81, but this is wrong for two reasons. Looking closely you will see that the green is not the main color, and is instead integrated with the brown and beige in approximately equal portions. Secondly green is a dark tone, and will make the subsequent colors dark. This is why I follow a beige, brown, green process.
So here is the step-by-step tutorial:
I basecoat the figure with flat white.
I hope this has been helpful.