jQuery Darken the background color of an element

jQuery.fn.darken = function() {
  $(this).each(function() {
		var darkenPercent = 15; // darken color by 15 percent
		var rgb = $(this).css('background-color');
		rgb = rgb.replace('rgb(', '').replace(')', '').split(',');
		var red = $.trim(rgb[0]);
		var green = $.trim(rgb[1]);
		var blue = $.trim(rgb[2]);
				
		// darken
		red = parseInt(red * (100 - darkenPercent) / 100);
		green = parseInt(green * (100 - darkenPercent) / 100);
		blue = parseInt(blue * (100 - darkenPercent) / 100);
		// lighten
		/* red = parseInt(red * (100 + darkenPercent) / 100);
		green = parseInt(green * (100 + darkenPercent) / 100);
		blue = parseInt(blue * (100 + darkenPercent) / 100); */
		
		rgb = 'rgb(' + red + ', ' + green + ', ' + blue + ')';
		
		$(this).css('background-color', rgb);
  });
  return this;
}
$('table tr td').darken();  // run the above plugin on all table cells