Polarization basically means that light can only pass through in certain directions. I can't say exactly why colors change and whatnot (off the top of my head), but it has to do with the windshield only letting certain wavelengths through, and the same with your glasses, adding up to a purple color. Also, ever noticed what happens when you tilt your head while looking at an lcd screen with polarized lenses on? The LCD also has a polarizer, so when your lenses match the polarizer in the lcd, the screen disappears.