Does everybody know about this? I came across it by accident a few years ago, and subsequently forget all about it until this week. It's an effect that only works with some data projectors - mainly those built by Epson, as far as I can tell - and it combines a wonderful demonstration of polarisation with colour mixing.
Basically what do you do is what is included in the video below: hold a polarising filter up so that light from a projector passes through it and hits the screen, and then rotate it through 90 degrees. I presume the details might vary from one model to another, but you can see what happens with my set-up. In one alignment, the light hitting the screen is green, in another its magenta. In between you get various shades of each.
Obviously the light from the projector is polarised, but different colours are polarised in different directions. When we see it all together - without filters - they combine to create white. Now why the projector does that is a different question. I remember emailing Epson when I came across this first, and receiving a long and detailed reply. The only problem was that they had done me the honour of assuming I would understand the finer points of optical physics and that is an honour I clearly didn't (and don't) deserve: all I could really follow was that they didn't set out to do it this way deliberately - it was more of a by-product of other processes.
You can add to the fun by opening up blue, red or green rectangles in Microsoft Word, or whatever, and then shining those coloured images through the filters.