h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

I am colour-blind. I tried (again) the Ishahara test http://www.toledo-bend.com/colorblind/Ishihara.html and could see the top left number, and -when told the right answer, using some imagination- the 2 further that the author of the page could see (I can see there's something, but what?...), and none other.

OK, doesn't matter. The funny story concerns my granddad. He was colour-blind, even worse than me, and once had to go and buy some violet thread for his wife. When he came to the shop and asked for a violet thread (I think my granddad carried a sample), the mercer said something along the lines of "Yessir, as you wish; but may you please help me, you see I'm colour-blind" . So they both tried together and eventually my granddad came back with a thread which had a very nice shade which would have fitted perfectly to my granny's violet thread, only it was not violet at all so the lady had to go by herself anyway.