How to dynamically change the color of progress bar background android

I'd like to dynamically change the background color of a progress bar in android. I followed the "bonus" part near the end of the page of this tutorial:

http://colintmiller.com/2010/10/how-to-add-text-over-a-progress-bar-on-android/

It changes the color, but only once. If called more than once, the progress bar disappears. Here's the code.

Here's the progress bar definition:

<ProgressBar android:id="@+id/progress_bar" android:layout_width="fill_parent"
    android:layout_height="wrap_content" style="@android:style/Widget.ProgressBar.Horizontal"
    android:layout_marginRight="5dp" />

This is the drawable definition in res/drawable/green_progress.xml:

<?xml version="1.0" encoding="utf-8"?>
<layer-list xmlns:android="http://schemas.android.com/apk/res/android">

<item android:id="@android:id/background">
    <shape>
        <corners android:radius="5dip" />
        <gradient
                android:startColor="#ff9d9e9d"
                android:centerColor="#ff5a5d5a"
                android:centerY="0.75"
                android:endColor="#ff747674"
                android:angle="270"
        />
    </shape>
</item>

<item android:id="@android:id/secondaryProgress">
    <clip>
        <shape>
            <corners android:radius="5dip" />
            <gradient
                    android:startColor="#80ffd300"
                    android:centerColor="#80ffb600"
                    android:centerY="0.75"
                    android:endColor="#a0ffcb00"
                    android:angle="270"
            />
        </shape>
    </clip>
</item>
<item
    android:id="@android:id/progress">
    <clip>
        <shape>
            <corners
                android:radius="5dip" />