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

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.