An indigo blue is a colour associated with several Japanese Army aeroplane types and indeed JAAF paint colour #28 'Sei Ran Shoku' (Indigo Blue Colour) is a dull, dark indigo blue. The manual specified that the camouflage paints to be used on the Ash Green base coat of the Type 97 were yellow brown, indigo blue and green. On other types the blue was sometimes applied as a thin dividing line between the green and brown segments but a photograph reveals that the 24th Sentai may have applied it in the manner shown above.
The scheme is usually depicted as two greens and a brown but we like this interpretation better!
Image credits: ©SFM 2008, ©Bunrin-do FAOW #29 1991; Colour information: Katsushi Owaki; Rendered colour chips: ©Straggler 2008