You are welcome, vampirella.
I'm glad if my information may be useful for you.
The rocket fighter "Shusui" project was a joint-development of army and navy.
J8M1 was navy's designation. For army, it was Ki-200. The book identifys the light green Shusui as Ki-200, so I think the color would probably have been for the army service.
The author, Toshihiko Ogawa, also says that the color definition was not necessarily so strict as the allies. Therefore, he often found out different green or grey on the same model of aircraft during the war. He says these colors in the book are based on his 'own' experience during the war and research after the war.
Thanks.