Wheee, bumping an old thread!
I just realized, I think Randall took some creative license with dead sub-pixels to make this comic. An actual single dead sub-pixel wouldn't result in a red or green pixel, it would result in a cyan, yellow, or magenta pixel.
Here's a normal LCD display close-up, and normal scale:
(Each pixel consists of a red, green, and blue sub-pixel, in that order. Hence, this image is 5x5 pixels.)
Here's what a normal dead pixel looks like, and how it would appear on your monitor:
And here's what normal dead sub-pixels look like, and how they'd appear:
(You might need a magnifying glass for this one.)
But on the comic, to produce a red or green pixel, TWO sub-pixels have to be out, like this:
For that to happen, two sub-pixels would have to die together in one pixel, and I wouldn't think that would happen very often. Even changing the arrangement of the sub-pixels (RBG instead of RGB, for example) wouldn't help. Even worse, the sky would likely be some shade of cyan, which wouldn't have any red in it anyway.
But then again, I suppose a yellow or cyan pixel would be a lot harder to spot in the comic.