<style>
.x-form-cb-label{
    color:red;
}
</style>

------解决方案--------------------

根据checkbox的id用脚本修改:
Ext.select("label.x-form-cb-label[for=checkboxID]").item(0).dom.style.color = 'red';

HTML code

<link rel="stylesheet" type="text/css" href="extjs/resources/css/ext-all.css" />
<script type="text/javascript" src="extjs/adapter/ext/ext-base.js"></script>
<script type="text/javascript" src="extjs/ext-all.js"></script>
<script>
Ext.onReady(function(){
    oForm    = new Ext.FormPanel({
        renderTo:"form_panel",
        layout:"form",
        hideLabels:true,
        border:false,
        items:[
            {xtype:'checkbox',id:'x1',inputValue:'x',boxLabel:'水稻'},
            {xtype:'checkbox',id:'x2',inputValue:'x',boxLabel:'小麦'}
        ]
    });
    Ext.select("label.x-form-cb-label[for=x1]").item(0).dom.style.color = 'red';
});

</script>
<div id="form_panel"></div>

------解决方案--------------------

JScript code

<style>
    .required .x-form-item-label {font-weight:bold;color:red;}
</style>

增加属性
itemCls:'required'