On the acctual car they are carbon fiber, so if you are not using carbon fiber decals then I would paint them semi-gloss black.
Here is a good reference to a Brabham BT49...not exactly the same car(differnt engine) but body materials is mostly the same, i think
http://www.gurneyflap.com/bt49-photo1.html
under "side pods" there is a good picture of the under side of a side pod
~G