JavaScript to change border color when the field value is changed from previously entered value

[PCForm]

JavaScript to change border color when the field value is changed from previously entered value.

var a=this.getField("Loss History")

var b=this.getField("Type")   this is a checkbox with export value "Amend"

Scenario: On renewal the value of this field is $30.  Midterm the user changes the value to $17 (change border color).

if(b.value=="Amend") and the user changes whatever value is currently in a.value, then I want the border color to change to show what had changed on the form.