Hi all,
The answer is . . . it depends!
Take a look at the photos from the
Memorial Flight's web site (they're a museum in France that builds flying replicas of WWI planes and restores originals to flying status). Click on each of their planes and you'll see a wide variety of cockpit interiors, seats, and instrument panels.
Generally speaking, the interior side of the fabric coverign would be light tan or beige in color. This is what the fabric covering looked like before being painted. However, on German planes with lozenge camo, the lozenge will be seen on the inside of fuselage because it was actually printed on the fabric. You'll see this if you click on the Fokker D.VII.
The Spad 13 has no instrument panel, but rather a built-in shelf for the instruments.
The Blériot XI² doesn't really have an instrument panel either. The instruments are just clamped to the frames. Nieuports were built this way too.
Many WWI planes had wicker seats. The Nieuport 28 at the National Air & Space Museum -- Udvar-Hazy Center has one. You can see photos of its restoration (rebuilding, really) with some good pictures of the seat
here.
Be sure to check out
Mark Miller's fantastic 3D computer cut away drawings of WWI aircraft to get a good idea of the interior layouts of a number of planes.
There are some great photos of real planes and models at the
WWI Modeling Page. Try the
models and photo archives search there to see if reference photos are available for the plane you're building. Even just looking at how others have painted their models can be helpful.
Finally, don't hesitate to post questions at the Areodrome forums. The
models subforum is a great place to hang out!
I hope this helps.
Regards,