Hello Spiros, @fiveten
Here’s a link that is loaded with some useful information. Nick Millman operates this website, called Aviation of Japan. I whole heartedly encourage anyone who is even considering building anything Japanese aircraft related to please check his website out. Nick has done some considerable research on the various colors used. He has a goldmine right here !!! A person can literally spend days looking at the various articles that have been posted over there. He has taken the colors and digitized them using a scientific approach, so you can see exactly what he is talking about.
As a bonus, Nick is a really good guy too…………….. I have corresponded with him on several occasions. He also posted an article on my 1/48 Tamiya Ki-61 build.
Please follow this link, as it will explain the interior colors used in the earlier Japanese Army AF aircraft.