页面中加入

<style type="text/css">html {filter:progid:DXImageTransform.Microsoft.BasicImage(grayscale=1); }</style> 

如果还有部分flash没有变灰,可以尝试以下操作(仅以SWFObject为例)
 var swf = new SWFObject("*.swf", "amcolumn", "190", "70", "8", "#FFFFFF");
  swf .addVariable("preloader_color", "#999999");
  //变灰代码
swf  .addParam("wmode", "opaque");
swf  .addParam("allowfullscreen","true");
swf .addParam("allowscriptaccess","always");
  //变灰代码
swf  .write("flashcontent")