I've passed the RAF eyesight tests at Cranwell inc. colour vision and those tests do seem harder than I remember - although I could see all the numbers. I agree though, the blue 5 on the purpleish background is a toughie. If you're in a test just don't say anything until you are totally sure of what you're seeing.
It's probably to do with monitor settings and the fact that monitors screw your eyes up anyway.
It would be interesting to know if you can 'develop' colour-perception difficulties or whether it's something you're born with.
[This message has been edited by bow5 (edited 08 June 2001).]