Download these two images:
Open them up full screen (gqview for example) so that you can flick from green to red instantly. Now set the screen to green and stare at it for say 20 seconds. Without looking away switch to red. You will see 'pure red' for a moment until your green receptors recover. I would describe it as sort of slightly pinkish, and very intense. It works better with CRTs than LCDs, and presumably even better with 'lasers'. Paul wrote a highly advanced raytraced version of this which you can view directly in your browser!