One light blue (or green) marker and one bright red marker.
Place your red decoder lens over the paper. The red filter will make the red marker "disappear" into the background, while the blue or green text will appear dark (nearly black), making the secret message clearly visible. Alternative Method: Invisible Ink DECODE RED - Part 222:26 Min
Cut a piece of slightly larger than the hole and tape or glue it over the opening. This will act as your "secret lens" that reveals the hidden text. Write the Secret Message (5 Mins) One light blue (or green) marker and one bright red marker