Change shape solid color at runtime inside Drawable xml used as background

I've a Drawable xml file (background.xml):

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

    <item android:id="@+id/shape_id">
        <shape android:shape="rectangle">
            <solid android:color="#ffefefef" /> 
        </shape>
    </item>

</layer-list>

used by a LinearLayout:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:background="@drawable/background"
    android:id="@+id/layout_id"
    >

Now i need to change the shape shape_id solid color at runtime based on some conditions. How to do this?

Found by me:

    View v = findViewById(R.id.layout_id);

    LayerDrawable bgDrawable = (LayerDrawable)v.getBackground();
    final GradientDrawable shape = (GradientDrawable)   bgDrawable.findDrawableByLayerId(R.id.shape_id);
    shape.setColor(----);