[southeastern]

This is a simple yet brilliant idea. Now for an analog version...

[nosmokewhereiam]

You can put a blacklight on a timer and use multiple pen colors and shades of blacklight.

[hinkley]

You should be able to illuminate post-its with a color LED and get them to change color. White or metallic gel pen might make an interesting visual too.

They also have those glass signboards that you write on with regular or phosphorescent ink.