Richard, if you do a search you should find, as I did, that there is actually a lot of referrence in other forums on IJN vessels and thier colour schemes. There are a lot of unknowns but there are also a lot of usefull information.
Read through this thread to see what I mean:
www.j-aircraft.com/faq/naval_ship_colors.htm
The Yamato seems to be catered for a lot better with a couple of good books available, one in the Anatomy of the Ship series, but there is also a lot of information that would be relevent to the Musashi.
A usefull site to have a look at is:
www.shipcamouflage.com
They also have a lot of reference books such as the Yamato one in the Classic Warship Publishing series Number 25
I don't think you are ever going to find a definitive colour scheme for such a vessel as not only are there differing opinions of the colours used but the ships were actually painted differrent colours at differrent times.
Have a good search, there is stuff out there to give you usefull ideas and after all part of the enjoyment of building a model is the research.