May I suggest you find a copy of "Japanese Infantry Arms in World War II" by Ritta Nakanishi, ISBN: 4-499-22690-2. It is chocked full of illustrations of how things were carried, used, maintained, transported and associated crews. Items covered: grenade launchers, Anti-aircraft weapons, anti-tank weapons, Types 11 and 96 LMG; Type 92 7.7mm HMG; Type 92 20mm Automatic gun; Type II 37mm Flat Trajectory IG; Type 92 70mm IG; Type 94 37mm ATG; Type 41 75mm Mountain Gun and military swords. Illustrations are in color. IMHO, it covers most of what would be used in China, Burma and India theaters.
Perhaps the best reference in English is U.S. War Department "Handbook on Japanese Military Forces", ISBN: 0-8071-2013-8. It has it all, but in B&W photos. Logistical efforts (of which camo would fall under) is discussed. remember, in the Pacific, the Japanese moved a LOT of captured arty pieces to the occupied islands. Also, when researching the TO&Es, you will discover that it mostly goes out the window--Garrison Commanders could and did use any and all hardware and personnel they could get their hands on! Many organizations (like Naval Landing Parties) were sometime kept at over 1000% in manning and equipment! So the concept of "the Japanese doctrine was to use Yellow, brown and green paint in some directive is not what you will find. instead, you will fine directions on how to fortify island positions and local materials to hide emplaced equipment. Japaneese Garrison Commanders had a LOT of flexibility in such matters.
In the first book, the "metal" equipment is depicted as being a light olive---very much akin to Citadel's base "Death World Forrest" green. No examples shown of any disruptive camo painting applied. Again, not saying you will not find such, just that such attempts to apply paint in camo "scheme" seems to be limited to available resources, time and manpower to apply it with rather than any enforced doctrine.
I sure hope this makes sense and helps. I have seen even a bright yellow used to help disrupt the shape of Japanese tanks. Not a color "I" would have thought "useful" to try to get a tank to blend into it's background!!!(yet another next to it, supposedly taken in the same battle totally devoid of "yellow" and only had brown and green applied
same unit, too!)
Bottom line: Anything goes and you will be pretty much historical for the Pacific Island battles/campaigns. IMHO.