Access the caption of a textfield

Hello,
I’ve to create a form in which you can see if someone edited a textfield. This should be achieved in the following way: The user opens the form. Then the default value will be written in a hidden field. Then it will be compared the values of the textfields. If there is a difference the caption of the visible textfield should be painted in a red color. Is it possible to access only the caption of a textfield with javascript (akin to the way this.font.fill.color.value = "255,0,0";) ?
Maybe someone could give me a tip.