Some years ago this was a common misconception, probably started by someone seeing a badly weathered wreck of some very old color photos. There was a time when both Pactra and Humbrol made these colors.
The Rufe was manufactured by Nakajima. According to one of my references (IPMS Toronto), the topside green was Dark Green, FS14077, and the underside gray was Gray, FS26307 or FS16307. Although this reference states that each manufacturer had their own version of the standard Japanese aircraft colors, this gray appears too dark to me. I would use the standard IJN light gray that most of model paint manufacturers have, especially if I was going to build the early overall gray with blue-black cowl version. I don't think that anyone would give me an argument.
As a sidelight to this, there is question about the shade of this standard light gray. It seems that most IJN aircraft received a final coat of clear varnish or lacquer to protect the color coat from the sea air. This coat yellowed very quickly and many claim that the light gray ended up having a light tan or brown cast to it. [:0]
Pete