ContextMessageTextField is an improved TextInput? that allows developers to easly display error messages below the component using the contextMessage property.
You can also define the color (using errorContextColor property) and other stuff. Check API for more info

How to create an input Field:

<ui:ContextMessageTextField id="userName" errorContextColor="0xFFCC00" />

How to display error messages:

public function set userError(errorMsg:String):void {
   userName.contextMessage = errorMsg;
}

You can see it in action on the Model View Presenter sample http://agile.gnstudio.com/nabiro/wiki/ModelViewPresenter_in_FX3