请看看flash help>actionscript reference这一节：
Understanding ActionScript > Using the Actions panel > About error highlighting

In Normal Mode all ActionScript export incompatibilities are highlighted with a solid yellow background in the Script window. For example, if the Flash Player export version is set to Flash 4, ActionScript that is supported only by the Flash 5 Player is highlighted in yellow. The export version is determined in the Publish Settings dialog box.

翻过来就是：
在actionscript窗口中，当处于正常模式下时，所有在输出时存在兼容性的语句将被黄色背景高亮显示。例如：如果播放器输出为flash4格式，那么，仅被flash5支持的语句将用黄色高亮显示，而输出版本是由publish settings对话框决定的。