dynamic color change of bottom border only

I have a text box in a form that only has the bottom border shown.  That box gets activated and deactivated programmatically (using a function) depending on whether some other checkboxes are checked.  When the text box is active, it is set to access='open' and I want the bottom border to change from gray to black.

I have what I believe is the correct function code for generally changing the color of a border, shown below:

function

 ULineTBoxOn(MySubformNum,MyFieldRef){eval(MyFieldRef

+ ".access = 'open';");
eval("xfa.resolveNode('form1.#subform["
+ MySubformNum + "]." + MyFieldRef + ".ui.#textEdit.border.edge.color').value = '" + BlackCode.value + "';");
}

But this does not do anything to my particular text box.  I did a text where I enabled all of the borders of my text box and then ran the above function on that text box, and it changed the color only of the top border.

I see in the text box's XML code that the properties for each border edge are set one at a time, with the bottom edge being third, as shown below:

<field name="Docket_Year" y="90.17mm" x="147.3198mm" w="13.4239mm" h="6.35mm" access="readOnly">

<ui>

<textEdit hScrollPolicy="off">

<border>

<edge presence="hidden"/>

<edge presence="hidden"/>

<edge/>

<edge presence="hidden"/>

<?templateDesigner StyleID aped1?></border>

<margin/>

</textEdit>

<picture>99</picture>

</ui>

What seems to be happening when I run my function shown above is that I am only changing the color of the FIRST edge referenced in the XML, which is the top (?).  Or something like that.  How would I change the color of the BOTTOM edge only?

Thanks,
Emily