@lgardner
It appears that it was a mix of underside colour, bare metal or silver paint, depending on the plane or when it was built.
In the first photo, you can see that the gear bays are either bare mental or painted silver. This plane (J2M3) was built at the end of the war, by, I think Takaza Naval Arsenal, so late war shortcuts were taken. Note also that the camouflage demarcation line under the cowl is different than usual. Instead of ending in a straight line at the front of the cowl, it is a curve, which is indicative of the Takaza arsenal (Mitsubishi built planes had straight lines here). Note also the upper landing gear legs are black (semi gloss bleak), as well as the small door yokes, where as the lower leg is bare dull aluminum. Another detail is that the small gear leg door looks to be natural metal while the mid gear leg door looks darker (I've read that some were painted in a blue grey primer, maybe this is it).
The second image is of the turbo version J2M4, which I think was a converted J2M3. the LG legs are a bit more clear, the shine of the upper leg is quite apparent. the gear bays and doors look to be dull aluminum, compared to the exterior of the gear door. Note also the placard on the upper gear leg.
The third image is of another J2M4. Here you can see that the upper leg is bare aluminum with black bands. The gear doors looks to be either underside colour or bare metal.
Fourth image shows a J2M3 with the inner gear door colour looking "similar" to the underside colour, although its hard to tell.
I'd say that early J2M3 would follow Mitsubishi's pattern of painting the gear bays and doors in the underside colour, and then as the war progressed, they moved to bare metal, or silver paint or blue/grey primer in some cases.
Nick Millman has a pdf on the colours of the J2M available thorough his site.
4 attached images. Click to enlarge.