So, um, why does duplex need dye?

Canon had to program it that way because of ink drying problems using
duplex. In Duplex printing the printer feeds the paper in and out
several times. You can't have wet ink going back into the printer -
prints and rollers will smudge. Even with faster drying dye ink the
process is slower than molasses in winter. Far easier and up to 3 times
faster if you print one side of the documents, flip them over and then
print the other side (or odd pages and then even numbered pages). Duplex
printing, in case you didn't know, actually uses all the colors to make
that "black", which isn't quite black, it's more like a dark grey.
Compare the two (pigment and composite dye black) and the difference is
quite big, and I find not acceptable. I won't use duplex if my life
depended on it. I tried it once. Never again. Wear and tear on the
machine and a useless waste of good color ink.

-Taliesyn

That would explain why the printer would use dye inks for duplex. But
why would the iP4300 use color dye inks to make black? It has a black
dye tank right there. Are you saying that printers without black dye
ink make duplex black from colors?