There's also the risk that relevant information is communicated only via variations in color which is bad for color-blind users.

The main concern here would be accessibility, I think. I for one already have some difficulty with some links here since they aren't visually distinct from the surrounding text other than a slightly different color. (As an aside, could you reformat your post to add context to your points? E.g. point 5 relies on us knowing about the question referred to)

As for the fourth point, that comment was in the context of lacking underlining capabilities. We can totally underline stuff here, so there's no reason to resort to coloring

"If you're quoting a long text that already uses bolds and italics, then all formatting tools have been exhausted". You are forgetting about blockquote. That sets quoted text apart visually regardless of font, size, boldness, or color.

Your first point isn't really solved by colours, as if they are allowed, the quoted text can also use them.

BTW, I consider the example linked to from your second point to be less readable than it would be with bold “Pro”/“Contra”. The colours don't draw my eyes to anywhere.