Thanks for the help but the hidden state doesn't seem to work. I've circumvented the problem atm by placing a blank text field behind the drop down box and just have the box control the fields color, turning it invisible for when I want the transparency.

Code example below. The "1" value is what the box starts at with a transparant/no fill and what I want to change back to that transparent/no fill when selected.var vName = this.somExpression;var fieldObj = xfa.resolveNode(vName + ".ui.#choiceList.border.fill.color"); if (this.rawValue == "1") {     ...

I've made a javascript to change the fill color of a drop down box depending on what is selected. Everything is working as expected except that I can't find how to fill it with a transparent/no fill color. I've found very discouraging info about being able to do this online, but the box starts as tr...