If you look at a blue OHP pen at a small angle it looks reddy purple, because it gets rid of the red by reflection...
You could set up an OHP slide with pens on it and light it right. It should go well with a colour mixing demo..
Thin gold films look red in reflection and blue in transmission as well
http://www.chaosscience.org.uk/dem/public_html//article.php?story=20031110172342163