Chaining classes doesn’t actually count (or look out for) n classes in your markup

Right, but why would it? Chained selectors is basically an AND question, i.e.:

section#this.that

is any section with an id of this AND a class of that, so

section.colour.colour

is any section with a class of colour AND a class of colour so of course it doesn't match class="colour colour" as having two colour classes.

Anyway, that's a silly way to do things.

the answer to a question no one should ever ask.

This is how you get away with !important —without using important.