Japanese researcher A6M232 has posted images of the Ki-27 flap in his possession at Fuku's Forum.
The blueish-grey and slightly greenish appearance of the paint (close up) is apparent. In the second photograph pages from the Japanese Paint Manufacturers Association (JPMA) colour standards are being held against the artifact. The JPMA deck cross-references each colour to the Munsell standard.
Dr Yasuho Izawa described the finish as light grey overall "this was the factory finish and, in my opinion, it is light grey blue plus a drop of yellow".
Image credits: A6M232 & Fuku BBS