How to make the fill color disappear automatically when text is entered.

Hi, 

I'm wondering if someone can help me with the text field properties.  My ultimate goal is to have the text boxes in my fillable PDF all be yellow until text is entered.  Once anything text is entered in the text box, I want the yellow to disappear.  I believe to do this I need to run a custom validation script but I am unsure what to entered to actually make it do what i want.  Any help would be appreciated!