I'm not sure I can give you a definitive answer, but my first thought was perhaps those slat undersides might have been painted green zinc chromate, or perhaps the yellow chromate. I found another modeler's build of this kit, and it appears this person went with the green chromate.
http://hyperscale.com/features/2002/images/sb2cpc_15.jpg
This is just one modeler's interpretation.
Another that I found in this next link, read to where he talks about lack of actual color photos; he goes with dark sea blue.
http://web.ipmsusa3.org/content/sb2c-4-helldiver-0
If I was doing one of these (and I have one in my stash), I would likely opt for the zinc chromate green if for no other reason to break up the potentially monotonous dark sea blue.