Thanks for posting the curves. It is not completely clear to me which
ones go together in a family, but from what you are saying, it sounds like
the "color controls the high frequency boost at 10 kHz, and then there is
a separate "mid frequency" equalizer control that controls the bump in the
4 to 5 kHz area?

The color (Blue, Red, Yellow, or Green) determines the shape of the
pre-emphasis curve, and the user can also select the total maximum
boost (at 10 kHz) to be applied. The Optimod 9100B does not provide
any other audio tone/equalizer controls, except for an optional bass
boost feature.

The 9100B's total audio bandwidth can also be adjusted, of course.
With no "brick wall" filter enabled, it provides a maximum audio
bandwidth of about 12 kHz. Western Hemisphere models can select
either a 5 or 10 kHz filter. European and Asian models have a choice
of a 4.5, 5, 5.5, or 6 kHz filter.

Meanwhile, the mono Optimod 9200's bandwidth is continuously variable
(in 0.5 kHz steps) from 4.5 to 9.5 kHz, and it offers a range of
pre-emphasis curves and boosts very similar to the 9100B's.

Since when was this the case? The equalization for all these media is
adjusted on the source side to suit what the originator thinks is best.
Why should AM radio be any different?

Because there has to be a level playing field. Why do you think the
RIAA curve for phonograph records was established? Because there was
too much variance and too much confusion, and it was inhibiting the
goal of providing the consumer with increased fidelity and
convenience.

This also applies to AM radio. Whether it be today's 10 kHz or the 5
kHz that IBOC proponents want, consumers will never get to experience
analog AM radio at its full fidelity unless there is a level playing
field and a universal standard which manufacturers can design their
receivers to meet.