To disable colouring in your current buffer, toggle font-lock-mode off with this command (my ad-hoc solution):
To disable font-lock-mode “permanently” – see above!
You may still find coloured remainders – but it’s not as bad any longer.
One day I suddenly noticed what should have been obvious from the beginning (in that environment):
$ echo $TERM xterm-256color
Now it was clear how to disable colours effectively in a non-GUI Emacs:
$ env TERM=xterm emacs