Changing the fillColor may not give you the results you are looking for: When a user has the "Highlight Existing Fields" function selected, the fields that are not active will always look blue. Only when you click in such a field, will the background color be shown. What I do is in addition to changing the background color, I also change the border color via the strokeColor property. The border stroke color will be shown even when the highlighting feature is active.
Karl Heinz Kremer
PDF Acrobatics Without a Net
PDF Software Development, Training and More...
http://www.khkonsulting.com