返回css样式，不是直接操作原始数据添加html代码

<style>.red{color:#f00}</style>

        viewConfig: {
            getRowClass: function (record, index, rowParams, store) {
                if (record.get('headDepartment')=='小计') {
                    return 'red';
                }
            }
        }

getRowClass( record, index, rowParams, store ) : String
Override this function to apply custom CSS classes to rows during rendering. This function should return the CSS class name (or empty string '' for none) that will be added to the row's wrapping div. To apply multiple class names, simply return them space-delimited within the string (e.g. 'my-class another-class'). Example usage:

viewConfig: {
    getRowClass: function(record, rowIndex, rowParams, store){
        return record.get("valid") ? "row-valid" : "row-error";
    }
}
Parameters
record : Ext.data.Model
The record corresponding to the current row.

index : Number
The row index.

rowParams : Object
DEPRECATED. For row body use the getAdditionalData method of the rowbody feature.

store : Ext.data.Store
The store this grid is bound to

Returns
String
a CSS class name to add to the row.

你这边有点误区，这个getRowClass 是让你返回一个 class 所以 所以你最好 用css 定义一个样式 在通过这里返回 ，得到这个样式后 会自己去渲染