Here's a useful website for interior colors of many US aircraft. The link below will take you to the F4F.
www.ipmsstockholm.org/.../stuff_eng_interior_colours_us_part3.htm
Unfortunately, it doesn't directly address the issue of the gun bays.
In part one of the article, author says that FS 36440 is a close match to Grumman Gray. This would correspond to Testors Flat Gull Gray in the Model Master line.
It's hard to tell from a b/w photo, but this link shows a Wildcat with both gun bays open. To me, they don't look bright enough to be zinc chromate yellow or even NMF. But who knows?
commons.wikimedia.org/.../File:Testing_machine_guns_of_F4F_Wildcat_aboard_USS_Enterprise_(CV-6).jpg
If you do a google search for images, you can see that some modelers have done up the bays in zinc chromate green or yellow; what appears to be a Grumman Gray; and other indeterminate colors.
With so much uncertainty, you're probably safe to go with any of these.
Good luck and welcome to the Forum!