Took me a moment to understand what's ironic, then I got it: the page discusses to use other visual (and also important, accessible text!) hints which don't solely rely on color, to convey information, besides purely color-coded information.

But then, the book uses solely the colors red and green to visually convey bad and good examples.

The meta-irony here is that reitzensteinm's imgur image cannot be understood by any visually-impaired person ;)

Oh wow. I wanted to be a bit vague so people had fun catching it. The meta irony didn't remotely occur to me.

That made my morning, thank you!

One delightful HN moment! :D

I'm missing the irony. Isn't the point that you need the arrows from the example on the right?

Because the colors on the top of each box indicate which are bad and good. Red for bad and green for good. But if you're color-blind (or on a b&w ereader) you can't see the distinction and have no idea which is the right one at a glance and this section is specifically about not using only colors to indicate information.