I am concerned about the reasoning here. You appear to be treating the two-colored dust as a phenomenon that demands an explanation, and demands it to the degree that we must reject any theory if it does not provide a perspicuous explanation for the two colors of dust. From there, you propose as an explanation [something about foam], rather than dust, and back it up with a tiny dust sample that has not been analyzed (or have you analyzed it now?). So the dust was two colors because it wasn't dust but foam, which was created by [TBD] from different parts of the building which made different colors. I like the novel approach, but I doubt inference to the worst non-explanation is going to catch on.
Furthermore, even assuming for the sake of argument that there are two distinct colors of dust (rather than dark smoke + light dust), that by itself means nothing. If we found no explanation for the two colors, I do not even think that would qualify as a mystery, puzzle, or oddity. To me it is like trying to figure out why a particular bit of ground had 1% less dust than nearby ground. We do not need a full explanation for that. Imagine someone saying, "We fully explained the collapse, but we don't know why the dust was colored the way it was." Sounds fine to me. But we can explain the colors anyway, I think.