Transparent Fill for Dropdown Box

i've made javascript change fill color of drop down box depending on selected. working expected except can't find how fill transparent/no fill color. i've found discouraging info being able online, box starts transparent (imported document preexising pdf has box background @ no fill) i'm hoping still possible. using cs3, appreciated.

code example below. "1" value box starts @ transparant/no fill , want change transparent/no fill when selected.

 

 

 

 

 

var vname = this.somexpression;

var fieldobj = xfa.resolvenode(vname + ".ui.#choicelist.border.fill.color");

 

 

if (this.rawvalue == "1") {

     fieldobj.value = "255,255,255";

}

else if (this.rawvalue == "2") {

     fieldobj.value = "255,255,255";

}

else if (this.rawvalue == "3") {

     fieldobj.value = "250,250,0";

}

else if (this.rawvalue == "4") {

     fieldobj.value = "0,145,0";

}

else if (this.rawvalue == "5") {

     fieldobj.value = "205,0,0";

}

else if (this.rawvalue == "6") {

     fieldobj.value = "90,90,90";

}

else if (this.rawvalue == "7") {

     fieldobj.value = "25,25,25";

}

else {fieldobj.value = "0,0,0";

}

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