Well the functionality for validation can still be used for your needs. If you change a little bit the code, just to change all the field's color, you don't need to verify if validation script returns a null field, you can just print once the validation code has run.
The validation script will look for all fields with null value and change their borders to red as needed.
I don't think it would be possible to create a class just like in HTML for all objects you want to apply fill color like CSS code, you would probably need to apply this with JavaScript when the print event is fired.
Otherwise, maybe another way would be having a function like ChangeColorField like in my PDF and call that function on pre-print event of each fields you want to change the fill color.