[ksmiley]

Pardon my ignorance, but why wouldn't this work?

Suppose you were red-green colorblind, and there was an apple tree with green "Granny Smith" apples outside your window. You can't tell if the leaves on the tree are red or green, but using common sense, you guess that they are green, and color them with the green pencil. You can't tell if the apples are red or green, but using common sense, you guess that they are red, and color them with the red pencil. "Ah ha", says your friend, "you colored the apples with red when they are actually green. You must be red-green colorblind"

[vhf]

That's how they discovered I was colorblind :

When I was in kindergarten, the teacher gave us the assignment of drawing a brown bear. Mine was green.

[albertoavila]

Some color blind people are more affected than others on their same color blind group.

In my experience doing an Ishihara test (http://en.wikipedia.org/wiki/Color_perception_test) whith someone else while telling them what are you seeing on each pod does wonders to make them understand specifically what shades of color cause you troubles, that's how my wife got to understand my color blindess.

[untog]

Sorry, I edited my post to reflect the fact that I misrepresented the scenario. Though it started as a discussion about colour-blindness, it developed into a more general "what if my blue is your red" discussion. In that scenario, the red and greens on the tree would be matched exactly to the reds and greens on the pencils- there's no way to know if I'm seeing something different.