Ignore all advice regarding how LEDs made in some specific colors are more efficient than others; just check the actual numbers. Which will take us directly to the point:
LED choice is by far the most important! Look at as many distributors and datasheets as you can find, sort for the best luminous intensity. There might be easily 50x difference between the worst and best LEDs, in such case it is totally secondary if you can increase your driver efficiency from 55% (resistive dropper) to say 90% (very well designed mini-buck per segment).
Now you can ignore some colors due to the sensitivity curve of the eye, but this is wasted effort because eye sensitivity curve is already baked in the lumen and candela numbers! Following wrong forum advice you might end up ignoring a really efficient 625nm red LED just because it's "red", so that eye sensitivity curve gets taken into account twice by accident, first by the manufacturer then by you again.
Also, is there really a reason to run the microcontroller at 3.3V? Most low-power MCUs run at lower than this, and while doing so, also consume less power. 2 AA cells range from 2V..3.4V; many MCUs can run directly off this supply. With average battery voltage being a tad below 3.0V or so, you would already gain extra efficiency in the simple resistor solution; some 20% improvement from lower drop over the resistors, and another 20% from skipping the boost . Low-end 2.0V is still enough for red LEDs with Vf around 1.6V; while multiplexing anyway, you could increase the duty cycle (on-time proportion) based on measured battery voltage; which you likely measure anyway with the MCU - which again would be easier if you did not have an unnecessary boost converter, as most MCUs can easily measure their own supply voltage.
If the sensor requires 3.3V to operate, I would look for a replacement part which operates over the 2V..3.4V range, getting rid of the boost converter would be a huge help. If this is impossible, then running only the sensor off the boosted supply, and level-shifting the communication to sensor (if necessary) is possible.
TLDR; invest your design time in:
* Finding highest luminous intensity LED display,
* Finding an MCU which operates directly of 2xAA
* Finding a replacement sensor which does the same, if unable, limit the boosted supply to sensor only and level-shift communication