// Here's the lines I used for notitication colors in my config.xml:
<config-file parent="/resources" target="app/src/main/res/values/colors.xml">
<color name="custom_primary">#FF007CD3</color>
</config-file>
<config-file parent="/manifest/application" target="AndroidManifest.xml">
<meta-data android:name="com.google.firebase.messaging.default_notification_color" android:resource="@color/custom_primary" />
</config-file>