With the policeman idea, perhaps you were going into the airport and the security guard found a knife in your pocket after you beeped going through the scanner...and you talked yourself out of getting arrested by doing a bit of magic...the color blind idea could also be incorporated into this theme, which is practical and relevant to your audience.

I sometimes say (of course I mostly perform for adults)

Now, I have a white pocket knife, but what is the opposite of white...or what Michael jackson used to be??

*change* Black, that's correct...

then do my patter from there but that's what I use to start it off

How about this patter...

A secret service agent working for the government (white) actually has a double-alias. He is also working for the rebels (black) who plots the downfall of the corrupt government. So, in silent battlefield of reconnaissance and intelligence dominated by high-tech gadgetery, stealth-cloaks and satellite cameras, the humble knife was used to identify their own troops - and their enemies. And it is because this person is a double-agent that this war is waging for so long - secret information is found, stolen, but thanks to the turncoat nature of the agent, information is often intercepted before it could reach headquarters. And so no side really has the advantage to turn the tide of war.

Little do we know that the agent in reality acts for a third-party (perhaps red?), a higher master, the ultimate puppeteer who lavished on the spoils of war and was the one who orchestrated the entire fiasco. Finally, when both the government and the rebels are morally and financially depleted, it was an easy task for the pupeteer claims the lands for his own.

Enjoy!