The
j-aircraft site has lots of research articles on Japanese aircraft. The article on "Zero Camouflage Schemes" should be useful to you.
Zeroes were made by both Mitsubishi and Nakajima and both companies used their own shades of green, grey and black. Nakajima-built aircraft began arriving at the front sometime in 1942 IIRC. In mid 43 the IJNAF added green uppersides to the factory standard A6M scheme of all over grey with a black cowl. Prior to this many units had already added ad hoc green camo using whatever paint they could get hold of.
For factory painted green and grey Zeroes there is a way of telling which factory made it. Mitsubishi-built aircraft had the fuselage camo division running in a straight line from the rear of the wing to the tip of the tail cone. Nakajma-built aircraft had the division run from the rear of the wing curving diagonally up to the front of the horizontal tail.
These are the RGB colours I used for my A6M skins:
Mitsubishi: green=36/42/32 grey=120/116/87 black=26/26/40
Nakajima: green=45/58/49 grey=141/147/137 black=34/33/38
