No, fabric parts were never left unpainted, eventually painted silver to protect them from UV rays.
A-14 should have been already utilized in the '30s.
There are photos of the restoration of the I-153 available both on French sites and on the forum at
http://sovietwarplanes.com/board/index.php?topic=1000.0 (I think to remember light grey structures and medium grey inner faces of panels) , some bad bw photos of I-5 insides
http://sovietwarplanes.com/board/index.php?topic=1724.0, and perhaps some details unpainted or of other colors.
Images of I-15 bis insides are available on Squadron/Signal and other monographs.
Light blue and light grey could also have been applied during overhaulings.
Regards
Massimo
