You show a small coffin and three small mummies, each of mummies are a different color.
When any of the mummies are put in the coffin and the lid is closed, it is impossible for anyone to tell which color mummy is inside unless it is opened.
At this point you turn your head away while the spectator(s) put one of the mummies inside and close the lid.
Now without even touching the coffin you can impress and shock your audience, and predict the color of the mummy that is inside.
A cute close-up effect, and everything is supplied ready to perform.