As a companion to Five Reasons Everyone (and I Mean Everyone) Should Care About Accessibility in Knitting and Beyond, let’s examine what the Internet can be like for people with a specific visual disability. Here, I’ve used the “Colorblinding” extension for my Chrome browser to show me what the Wikipedia main page looks like for people with different kinds of colour blindness.
First, we have the page as viewed by people without colour blindness:

All very straightforward, you can clearly see where all the text links are because they’re a different colour from the regular text, and the ones ringing the puzzle-piece sphere are even bolded.
If we switch to what the site looks like for someone with protanopia (red-blind):

We can see that the colours are a bit different, especially for most of the icons at the bottom, but we can still see where all the text links are.
Now, switching to what the site looks like for someone with tritanopia (blue-blind):

This is where things start getting more difficult for users. The coloured text links are now lighter. The ones ringing the puzzle-piece sphere are still easily readable, because they’re bold, but the text links across the bottom of the page are faint and more difficult to read, because the colour is so light. This user may not know where the links will take them, and may end up wasting time following links they’re not actually interested in; or they may decide not to visit them altogether, even though they may lead to features and information relevant to them.
Finally, we switch to what the site looks like for someone with achromatopsia (monochromacy):

For a user with this condition, they are probably not going to know that the text links across the bottom are even links at all, because they look almost identical to the regular text – virtually the same colour and size, and with nothing else to distinguish them, such as bolding or underlining. This user is going to miss out on some of the important features and information that the site offers.