Nav = {
    lis : null,
    initialized : false,
    colors: ['orange', 'green', 'blue', 'purple'],
    
    initialize : function() {
    	if (!this.initialized) {
            this.lis = $$('.nav > ul > li');
            this.assignClasses();
            this.initialized = true;
    	}
    },

    assignClasses : function() {
        for (var i=0, c=this.lis.length; i<c; i++) {
            var li = this.lis[i];
            if (!li.up('li')) {
                li.addClassName(this.colors[i%4]);
            }
        }
    }
}
Event.observe(window, 'load', function() {
	Nav.initialize();
});