> most colours are actively adding irrelevant information to the cognitive load of existing. It should be obvious that apples and red and the sky is blue.
That’s silly, because it does add relevant information. Obviously it’s a spectrum - too many colours can hide information, but when used appropriately it’s fine.
Also everyone is different. Perhaps your brain gets distracted by the colours more than the majority of people.
As you had guessed a little later, there are a few different emacs packages that do this. One of them is "rainbow parentheses" that gives every bracket a different colour (remember that emacs supports lisp, so differentiating between lots of different parentheses is arguably more useful in emacs than any other editor). [0].
Another one is highlight parentheses [1] which highlights all parens that enclose the cursor position, and gives a darker colour to those "further away" from the cursor.
Emacs' 'Rainbow Identifiers' does that. I like it.
https://github.com/Fanael/rainbow-identifiers
I can imagine more useful highlighting than color coding the types of the symbols encountered. Lighting up the active scopes. Giving the same hue to names that look like each other. There are probably highlighters out there that do that. But "simple" syntax highlighting is still the norm.