Hi Hiker,
Check this out:
https://www.intersil.com/cda/deviceinfo/0,1477,ICL7665S,0.html
You can use a ICL7665S to switch between to different
voltege levels: +5volt for the red anode(s) and +6volt
for the green anode(s). There are in fact two level
detectors and both are set to 5,5 volt which means
that around +5,5 volt both may be on or off but that's
no real problem because you switch from +5 to +6 volt.
There's no need to calculate a hysteresys because it
will only complicate the circuit and it's not really
necessary.
The highest voltage output will be connected to the
green anode(s) because the voltage drop across the leds
of the green display will also be higher. If this is not
enough to get the same brightness with the same cathode
resistors you can increase the voltage difference even
further. e.g. +5 volt and +7 volt. You only have to
recalculate the trip point for +6 volt.
The fets are old devils and you can use almost every old
mosfet you can get on condition that the maximum drain
current will not exceed 500 mA at +5 volt supply voltage.
on1aag.