The pigments are either in the raw plastic or the resins they just don't make molds then dye the product coming out. So the raw black material could come from a supplier in India and the raw white material could come from China, or they could come from the same plant, the composition of the batches even if made in the same plant could also be different. So your comment about not seeing how it could only effect one color is wrong, it could effect one color or both, or just a batch of one and not the other. The whites could be made with different mold, and have different dimensional tolerances than the blacks which could cause stresses during manufacture that causes the cases to crack. Until apple gets a good sample back and analysis what is causing the problem all these senarios are just speculation.
Nice to see comments from someone who seems to know a thing or two about plastics and manufacturing. Thanks!