Well, the green uniforms were not unheard of in the warlord period even if khaki and gray-blue were more common. I have seen a couple of colorized period photo of people in greenish hues, it appears that there were some sales of surplus American field tunics and trousers after 1919.
(PS: modern PLA is camoed for combat and the Class A uniform is a mixe between green and a bluish thing... they switched from the old green sometime ago, I think before my friends got in because they complained they have to change Camo but not class A).
Arrigo