API level >= 21

radio_group_bg_middle_child.xml

<?xml version="1.0" encoding="utf-8"?>
<ripple xmlns:android="http://schemas.android.com/apk/res/android" android:color="@color/fill_color_when_pressed">
    <item>
        <selector>
            <item android:state_checked="true" >
                <shape android:shape="rectangle">
                    <solid android:color="@color/fill_color_when_active" />
                </shape>
            </item>

            <item android:drawable="@android:color/transparent"/>
        </selector>
    </item>

    <item android:id="@android:id/mask">
        <shape android:shape="rectangle">
            <solid android:color="@color/fill_color_when_pressed"/>
        </shape>
    </item>
</ripple>