Thanks to Ed Yong over at Discover Blogs I've just found this rather strange, and quite tricky colour acuity test. It's not a test for colour blindness as such but more about how well you differentiate colours.
Try it. It'll take about 5 minutes.
Oh and I scored 7, which seems pretty good (0 is best; 100 is worst).
There's a lot more on colour vision and colour blindness on Wikipedia.