You can find everything you need to know about the interior colours of WW II US aircraft here: http://www.ipmsstockholm.org/magazine/2004/01/stuff_eng_interior_colours_us.htm
This is the cockpit of Gp Cpt Nowierski's Mustang HB886 taken Oct 1944.
Looks rather dark for wood.
As this picture from a comtemporary manual shows, the plywood was covered with a black antislip material:
After some wear and tear they probably looked more like this:
The Tamiya P-51B (as well as the iCM clone) has an 'incorrect' metal floor a la P-51A.
However, this photo is captioned as a P-51B and it has a metal floor.