I tried to design this without much success so any help would be appreciated.

A circuit where initially in a Bi-Color LED, the red led illuminates. Then it switches off and the green led illuminates, when an external voltage is applied.

for example: initial state: external voltage 0V -> red led on, green led off

external voltage changes to 5V (or any) -> red led goes off, green led turns on

if the external voltage changes back to zero it resets the leds to the initial state.