http://perplexus.info/solution.php?pid=3171

This generated some positive response. I though I would share how it came to be.

I teach high school math and have a new white-board on which I use colored markers. One morning in a geometry class I introduced a tool called a Mira. (picture: http://www.eaieducation.com/501012.html ) It is basically a piece of clear, dark, red plastic. It can be used to see both an image and its reflection when placed upright on a sheet of paper.

Every student who ever picks one of these tool invariably holds it up to see everything with a red hue. One of these students dicovered that some of the writing on the board was completely invisible - anything orange or red. Whereas anything blue, green, or black just looked black.

That was my inspiration. While my students worked diligently on their assignment (yeah right) I made diffent colorful pictures with words coded into them. These picture were usually an array of colored dots such that when one held up a Mira the right combination of dots were made invisible and a message was formed.

I wanted to make a similar puzzle for perplexus, but I knew a puzzle requiring colored filters was out of the question, so I decided to use even and odd digits instead.

yes... you should sell your idea