It's just easier to distinguish SE from meta.SE when one of them has a greyscale icon like a proper Meta should.

msegreyicon.user.js

// ==UserScript==
// @name Greyscale MSE Favicon
// @description It's just easier to distinguish.
// @namespace michaelb958
// @author michaelb958
// @license MIT (http://opensource.org/licenses/MIT)
// @include *://meta.stackexchange.com/*
// @include *://*.meta.stackexchange.com/*
// ==/UserScript==

function with_jQuery(f) {
  var s = document.createElement("script");
  s.type = "text/javascript";
  s.textContent = "(" + f.toString() + ")(jQuery)";
  s.setAttribute('data-with-jquery', '');
  document.head.appendChild(s);
};
 
with_jQuery(function($) {
  var icon = 'data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABAAAAAQCAYAAAAf8/9hAAAABmJLR0QA/wD/AP+gvaeTAAAACXBIWXMAAAsTAAALEwEAmpwYAAAAB3RJTUUH3gQSCAYSQIBpXQAAAPtJREFUOMu1krFKxTAUhr8kRXF0FB+k4CpOzk6dFBfBPklewEHolN2hT1FwFARXaaAUW6QtCEKcoqm31Fy4/pAhJ/zn5Ps5wjkHQFVVCXANHLKuDrhP0/TTOUfiq0KIB6XUOREahuEUuACQvqiUOosxj+PINE0/g5xzeIxtNUMoy3K1S9d19H3/fc/zXMwQtjFvfGMnCEVRbHRp25amaRbNWut1hDXz/yBorV+llMfWWqy1f+4ScAQMMtjE/UgzwAGwN8ugrusra20FPAbnGfj4ZX4BLo0xbwDC8wshFkdlWRYGdGOMuQszkJF5vQO3odkriWxwYox5Wnr4AsusgVErbYK7AAAAAElFTkSuQmCC';
  $('link[rel="shortcut icon"]').attr('href', icon)
    // doesn't take effect immediately in Firefox
    .detach().appendTo(document.head);
});