I can tell you first-hand experience from looking in the cockpit of a 1st FW 94th FS (Spads) at Langley last year that the color is flat black. Time and age has weathered it a bit, and you can mix in a little flat grey, but just a bit. Yes, it is an avionics bay, and was used for expansion of the "black boxes" for the aircraft over the years. It's still pretty empty back there though in a C model. Apparently it generates a lot of heat back there too.
I can also attest that this is the color for that area of the F-15Es at Mountain Home, but I cannot attest to the colors other squadrons might be painting in that area. There is the possibility that the color varies, but for the ones I've seen up-close-and-personal, they are flat black. I also remember alot of bundles of wires in that area. Istis for the most part an analogue aircraft after all!
I just went through my pics to see if I photographed that area of the cockpit, but I didn't have any. Much of the interior of the cockpit is also flat black (the instrument panels themselves), but the instrument frame (for which the panels drop into) is the same light gray as the outside of the aircraft (the lighter grey in the ME scheme).
Let me know if you need any other pics of the F-15C or E. I've some externals of the E (they wouldn't allow me to photo the E in the cockpit), and I have both internal and external of the C, including good shots of the engine outlets.
- Snacko