The interior color for those airplanes is Chromate Green.
There is also another color used on some pieces but I don't think it was used on B-17 interiors but was used on other airplanes in various areas like wheel wells and landing gear pieces--the color is called Zinc Chromate, which is a kind of a "bronze green". It seems that it is the Chromate Green that is predominate.
Go to the library and check out some B-17 reference books--there are many. One book, "B-17 FLYING FORTRESS IN DETAIL AND SCALE", shows late model B-17G waist section in natural metal (Boeing image number 1624). Also, Boeing image number 36835 shows a late B-17G waist section as natural metal. Many times the forward compartment is covered with material/insulation and is green colored.
I have several books that show B-17 interiors, but it seems that some late production B17Gs have no paint in the waist section or tail section. But it seems that the B-17E/B-17F and early B-17G have the interior painted Chromate Green. The interior of our C-47 is painted Chromate Green.
The Monogram/Revell visible B-17G instruction sheet should tell you what colors that the airplane is painted. Mine do (I bought this kit in 1979 when it first came out). I have built over two dozen of these kits since 1975 and I have three of them flying around my house right now.
My Monogram instruction sheet for the same kit indicates the following:
Floor: Zinc Chromate
Bulkheads: Chromate Green (except the bulkhead facing the waist section which is painted "aluminum" per the instructions with Chromate Green framing).
Instructions indicate in more than one panel:
"Basic bulkhead color is Chromate Green."
"Entire [fuselage] interior is Aluminum."
Your best bet is to follow the painting instructions that come with the kit. Monogram/Revell is known for getting it right most of the time. Also, you can't go wrong looking at as many reference books as possible. I don't know where you live, but any good size library has many aviation books and probably have at least one on B-17 Airplanes.
A small sample of the B-17 reference books that show the interior:
B-17 FLYING FORTRESS IN DETAIL AND SCALE
PILOT'S MANUAL FOR B-17 FLYING FORTRESS
THE B-17 FLYING FORTRESS STORY
B-17 FLYING FORTRESS AT WAR
GUNNER--An Illustrated History of WWII Aircraft Turrets and Gun Positions
The cover of the book GUNNER shows a B-17G waist interior painted Chromate Green. The same book shows the AF Museum B-17G Shoo Shoo Shoo Baby with an entire Chromate Green interior. This is the book you should look at--Many detailed color photos of the AF Museum's B-17G.
Chromate Green paint for scale aircraft interiors is available at any good hobby shop.
Good Luck with your B-17 model
Builder of over two dozen 1/48 Scale Monogram/Revell B-17 kits.