The one thing that you want to remember about the Rufe is that, although you will see drawings of it painted purple in several reference books, there is no evidence from any authoritative source that a Rufe was actually painted purple like the box art. The box art is a printer's error. (Confirmed by Tom Barker.) it was supposed to be orange.
The tail number, on the decal in the box, begins with the letter 'Ko' indicating that the aircraft belonged to the 'Koko Gijitsu Sho' or Air Technical Arsenal which was the Japanese navy's experimental testing center. Since the paint job for Japanese experimental aircraft and trainers, at the time, was all over orange with black cowling, the box art and decals would at least be in the ball park with an orange paint job.
Aside from the all over grey and green and grey color schemes that were used on the Rufe in combat you could also use the all over Navy blue with French markings used on captured Rufes flown by the French in Indochina after the war.
The Japanese sometimes put some extremely colorful personal and unit markings on the rear of the fuselage and tails of their aircraft, but , unfortunately, most of the available photographs are in black and white. The artists who have done color plates must have had the benefit of interviews with pilots, read pilot diaries or extrapolated the colors of the markings by the "lets see, the plane was green, the meatball was red, it was orthochromatic film so the color of the unit flash must have been..." method.
When you finish are you going to enter it in the 'G' Challenge?
http://groups.yahoo.com/group/guillows_g_challenge/
Howard