Thanks you for your answers, i am a bit farther but not yet arrived.
For now,I use the “trick” used in the shadow sample. a (shader) parameter named texDisable which is set to 1 for mesh that don’t have any texture.
The problem originally presented is gone.
But some mesh don’t have texture everywhere. Some of their vertices have only color information, the rest of them having texture info.
And I don’t see how to shade/manage this kind of mesh .
Maybe parsing the eggs and looking for the color info ? but then what ?
(Note that, for many reasons I cannot correct the meshes themselves).
( and yes, it’s a shame, that would be easier)