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

A circuit where initially a red led illuminates. Then it switches off and a 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.