Sure you can. You can do this during the Validate event, for example:
// Validate script
event.target.fillColor = event.value ? color.white : color.transparent;
thank you so much - that is exactly what I need!
One little tweak: when user enters text, along with making text box's fill white, how to make the text box border of a specified (or default if there is one) width to appear as well?
thank you for your time,
You would similarly set the field's lineWidth property. There are four possible values, as documented here: http://livedocs.adobe.com/acrobat_sdk/9.1/Acrobat9_1_HTMLHelp/JS_API_AcroJS.88.708.html
You can also set the border color using the strokeColor property the same way: http://livedocs.adobe.com/acrobat_sdk/9.1/Acrobat9_1_HTMLHelp/JS_API_AcroJS.88.723.html