I always carry my rainbow knife when asked to do some magic at a moment's notice. You show the rainbow knife on both sides, the helper picks a number between 1 and 6 which of course is always the red colour. You change the rainbow knife into a red knife , do a shuttle pass and give them the ungimmicked , red knife to examine.
I have strong magnets on a piece of cardboard in both of my back pockets so that I can ditch or steal the knives easily.
Darryl the Wizard