"E6 film (and any other than Kodachrome, for that matter) have the dyes in
the emulsion from the factory. Color is
removed as required in the processing."
Not exactly.
Most color processes (Polacolor and Cibachrome being the major exceptions)
use dye couplers, not dyes. These react with the exhausted developer to form
a colored dye. Polacolor and Cibachrome use fully formed dyes, which are
either transferred to the receptor sheet or destroyed if not needed,
respectively.
In the case of Kodachrome, the dye couplers are introduced during
processing, one at a time. Kodak originally tried to control which layer of
the film received the coupler for it (eg, cyan for the red) by diffusion,
which didn't work very well. They finally realized that all they had to do
was selectively re-expose the three layers one at a time, because the dye
coupler wouldn't react in layers where there was no exposed silver halide
for the developer to develop.
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