This is definitely a good idea, its called challenge and reply/response. Its used to identify a friendly. Heres an example of it in action. You hear someone enter the house without unlocking the door, its dark, your groggy, you grab your firearm and hold up in the room. You then call out "blue" and your teenage son who just snuck out for a night of partying without you knowing replies "Zebra" and you avoid a catastrophic event.