They may be using a low viscosity mix as I've seen metal catch basins blend in color to the drill area location - especially when the top layer is the reddish soil. It turns the basin fluid to a light orange color. In any case, the voiding and the "lift" of the perforated drains under the slabs would allow a volume of this drill mix material to accumulate without entering into the underslab drains. Only under pressurized "wash" conditions, when the spillway is in operation, will this residual volume of material be flushed out. The revelation of the whitish grey residue in the emergency spillway photo infers a link to the mystery of the "milky white" coloring that entered into the Feather River.