darkenDown = function darkenDown (el) {
        if (el !== null) {
            if (el.getAttribute('data-note-type') === 'white') {
                el.style.backgroundColor = settings.whiteKeyColour;
            } else {
                el.style.backgroundColor = settings.blackKeyColour;
            }
        }
    }
Revert key to original colour.
@param  {element} el DOM element to change colour of.