Transparent text fields in Acrobat

I've made transparent text fields in Acrobat. I've set them to have no fill or stroke so they should be transparent. I have already drawn coloured boxes where they are in Illustrator.

But the annoying thing is, I want them to be transparent once the text has been entered into the text fields. At the moment the user can type in the fields but the boxes remain a light purple colour. I want this purple to disappear as I have white text on a photo background.

I'm hoping this is a simple error on my part!!