set an item or change color to let us find the real panel

How to set minimum one element in ExtJs 3.4 panel or set background-color or other property which will let us know that we are working on right panel while displaying.

I tried this but it show no label in Panel

Code:

CustomPanel = Ext.extend(Ext.Panel, {
            constructor: function(config) {
            config = config || {};
            Ext.apply({
                    layout: 'form',
                    labelAlign: 'left',
                    labelWidth: 85,
                    style: 'background-color:red', // this doesn't work
                    defaultType: 'textfield',
                    width: 600,
                    height: 400,
                    items: [
                        {
                            xtype: 'label',
                            text :'fisrt label',
                        }
                           
                        ],
            }, config );
            CustomPanel.superclass.constructor.call(this, config);
        }
    }
    )

if we create an instance of this and set Style then it works

Code:

this.dotmailerCenterPanel = new CustomPanel({
                                        region:'center',
                                        title:'Some title',
                                        style: 'border: 2px solid green;',
                                        bodyStyle: 'background: red;',
})

set an item or change color to let us find the real panel