The standard camouflage colour for late war Japanese ARMY Air Force (JAAF) aircraft was Yellow-Green #7. This was a rather lurid olive drab colour, that could easily be seen as "brown" from some distance.
Please see:
http://www.aviationofjapan.com/2011/...-go-shoku.html
(This is indeed the colour of the Ki-43-III shown in your profile, although the artist's interpretation is far too brown.)
The standard late war Japanese NAVY Air Force (JNAF) colour was dark green or black green and depending on the manufacturer and various weathering conditions, this too could appear as "brown". You can easily see the effect in this colour footage, shot shortly after VJ Day.
https://www.youtube.com/watch?v=R6LEq48sz4o
Compare the dark green colour (Deep Green Black D1) of the Mitsubishi-built "Jack" at about 0:22 against that of the Nakajima-built Zero (Dark Green N1) at about 0:43. It's easy to see how this latter colour could appear as "brown".
As for the brown mottle, both late war camouflage colours were typically applied in solid coats. A possible explanation could be heavy paint chipping being mistaken for mottle. Or indeed earlier green mottles appearing more brown than when originally applied due to weathering.
Attached is my profile illustration of a heavily-weathered 9th Sentai "Tojo" that was captured at war's end. From Nick Millman's Ki-44 Aces of WW2 (Osprey Publishing).
