If your rear light already has a lens and you are comfortable assembling small components, another option is to use 2 pre-wired surface-mount LEDs. For less than 50 cents each, here are some 0402 size LED (0.04" x 0.02") or about 1mm on the longest dimension.
So you could fit 2 of these LEDs within the "footprint" of a typical 3mm sized LED popular in O gauge. If you have a lens in place you won't be able to tell the light is coming from two different bulbs. I don't know how prototypical dual color backup lights worked but they might have been implemented with 2 different bulbs too? Or was it a red lens that rotated into place over a white bulb sort of like a semaphore signal?
In any case having the wiring separate (rather than a 2-pin bi-color LED) allows you to wire it up for 3-wire common-anode or common-cathode operation which could simplify hookup depending on your electronics.
