Javascript code - change text box fill

I have 2 boxes - 1) Is a running total 2) Cash Avail.  I want to write code that will change the Running Total fill to red if the amount is above the Cash Avail

Here is what I have

var RT = this.getField("Run_Total");

var CA = this.getField("CashAvail");

if(RT.value > CA.value){

event.RT.fillColor = color.red;

}else {

event.RT.fillColor = color.transparent;

}

I also tried

var RT = this.getField("Run_Total").value;

var CA = this.getField("CashAvail").value;

if(RT> CA){

event.fillColor = color.red;

}else {

event.fillColor = color.transparent;

}

But nothing happens.  I am sure I am missing something.very simple