How do you change a textField 'background fill' to NONE at runtime

> this seemingly simple problem has been perplexing me forever..

I understand how to change the txtField.fillColor to any value but NONE / no fill

to further illustrate:

BLACK IS: txtField.fillColor = "0,0,0"

WHITE IS: txtField.fillColor = "255,255,255"

but what is NONE or NO FILL?

is there a different method/scripting property used to 'remove' fillColor??

i'm baffled - please help!