Through a thread here (in danish) I found this T-shirt. I'm actually slightly red/green challenged so while I could tell that there was a text there I couldn't tell what it said. (I can tell the colors apart easily but some of the tests designed to trip people like me up do trip me up). Asking a colleague produced no result (more danish) - well actually it did, he's just telling the better story where it doesn't - so I did the reasonable thing and wrote a perl script to color separate the image so I could tell what it said.
The color separation also shows how the trip up actually works. Above is the red, green and blue components of the image respectively. Notice how the blue component is just noise as far as the test goes. The red has more signal in the letters and the green has less signal inside the letters. One can see how the obfuscation works when you have trouble distinguishing red from green: The reduced green signal is matched by an increased red signal, but if you can't tell them apart this just cancels to noise.
Presumably, if the lowered green matched the heightened red exactly I wouldn't even know I was missing out on something if it weren't for the social clue in "this looks like one of those colortests".
The blue component has absolutely no effect on the readability of the text (for me, that is). This image, which is the red and green without the blue, is more or less as unreadable as the original (i.e. I can tell there's something to miss, but not what I'm missing).
The code I used for the separation can be found here.