just apply following line on style.xml
<item name="android:listDivider">your_color</item>
The below is just information for your knowledge …
If you have seen design support library .. they are using following layout for NavigationView seprator..
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="wrap_content">
<View android:layout_width="match_parent"
android:layout_height="1dp"
android:background="?android:attr/listDivider"/>
</FrameLayout>
here, you can see android:background=”?android:attr/listDivider” ..
So enjoy …
and here is my output that i change color to holo_blue