orange_health.user.js

// ==UserScript==
// @name         Orange Neopet Health
// @namespace    http://www.reddit.com/u/undergroundmonorail
// @version      0.1
// @description  yellow is hard to read. make your neopet's worrying-but-not-critical health show up orange instead
// @author       monorail
// @match        http://www.neopets.com/*
// ==/UserScript==

(function(){
  a = $('.activePetInfo');
  a.html(a.html().replace(/(<font color=")yellow("><b>\d+ \/ \d+<\/b><\/font>)/, '$1orange$2'));
})();

You can drop the regexes by leveraging jQuery a bit more:

$('.activePetInfo font[color=yellow]').attr('color', 'orange');

woah, sweet. i'd literally never used jquery before. good to know :)