this is a complex bug I hope someone will be able to shed light now. Right from the start, I can't provider any reproducible test case beyond supertuxkart - the irrlicht mesh view demo, for instance, uses no lighting and can't reproduce that.
First, here is what the object looks like in blender :
the red color is done using vertex painting. Okay a few vertices are not fully painted in blender but I fail to see how this could result in what we see in-game!
In game, however, here is what we see :
What we have found out about this :
1) it only happens when the silo is exported as one with the rest of the track, in a large model. If we export it as a separate scene node, the shading becomes correct again.
2) it occurs on the faces facing the sun. If the model is rotated, the new faces facing the sun get this.
3) If I reduce the power of the sun, it stops occurring.
4) tried disabling specular lighting, didn't help. fully disabling lighting makes this go away.
5) Note that the normals of the model are smoothed so even if the sun somehow managed to make vertex colors go away I think it should still look smoother than that
6) repainting the object differently tends to make the issue go away, like only specific colors were affected??
So I think that under some circumstances, unfortunately I don't know which, the sun overrides the vertex color on SOME vertices of some faces.
Anyone has a clue how to further debug this??
