[ydant]

Here's one I've played around with.

http://colorfilter.wickline.org/

I'm not really clear on how it or color blindness in general works. So I don't know if finding the filter which makes the image unchanged for me (color-blind) is a good way of representing how the world looks to "normal" visioned people.

[LaaT]

Thanks, that was it. No, it's not about representing how the world looks to normal visioned people, it's about how we cannot differentiate between some colours, or all colours for the worst case.