I looked everywhere for this information a few years ago.
I checked for color photos from the web, no luck.
I looked through every book I could think of for a reference or photo, again no luck (no color only non discript black and white photos).
I asked on different message boards, nobody could be sure.
I asked my brother-in-law's father (PT Boats in Pacific), he could be sure what color the barges were, but thought they were green or brown as they hid along the shore line to avoid aircraft, (it has been a long time).
I decieded that I would go with a Japanese Navy green, a very dark green.
A few months after I painted them up, I went to the Marine Corp Air/Land museum at Quanico, VA. Big display on Japanese landing on Wake Island.
Japanese landing craft were painted a light brown. I guess that the Marine Corp would have better references I was able to access. So I would go with the light brown.
I did not go through the process to repaint my landing craft and still think they look great in green.