Adobe Form Filling issue

Hi 

I want to be able to have the fillable form I have created in part greyed out once another part has been checked i.e. if I create an A4 field and an A5 field I need the opposing field to be greyed out once someone has checked either option, not both. In properties all I can see are 'actions' but based on 'mouse entering' etc and that doesnt seem to work once applied - am i looking in the wrong tools box as I only see properties for it.