Hi there,
I still believe it is some kind of camouflage - the contrast of the dark color beneath the light spot with the same spot and the visible part of the undersurface color is way to strong to be a shadow. Also the number "67" is not completely shaded by the darker color. All of this brings me to the conclusion that the dark color was there before the stars were overpainted and the chinese numeral was applied. I also saw that darker blotch beneath the canopy and yes, it might be also a part of the camouflage. On the other hand, the photo is of very bad quality
Someone's got a better one?
I also looked up the two books on Khalkin-Gol I have and one of them showed a picture of camouflaged I-16s. So maybe the Far Eastern regiments (from where the first I-16 in China originated, as A. Demin states in his article about soviet volunteers) applied camouflage even before the conflict with the Japanese, as this practice did not seem so uncommon to them.
-Christian