Hi,
This is the script I use. Note that you have to specify the UI reference for the object type:
var vName = this.somExpression;
var fieldObj = xfa.resolveNode(vName + ".ui.#choiceList.border.fill.color");
fieldObj.value = "255,255,255"; // white
/* Check the type of object and change the .ui reference:
Date field = #dateTimeEdit
Dropdown = #choiceList
Checkbox = #checkButton
Text field = #textEdit
Numeric field = #numericEdit
*/
Also the object has to have a background colour to start with.
Hope that helps,
Niall