I solved it by editing styles.xml:
<style name="ToolbarColoredBackArrow" parent="AppTheme">
<item name="android:textColorSecondary">INSERT_COLOR_HERE</item>
</style>
…then referencing the style in the Toolbar definition in the activity:
<LinearLayout
android:id="@+id/main_parent_view"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical">
<android.support.v7.widget.Toolbar
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:id="@+id/toolbar"
app:theme="@style/ToolbarColoredBackArrow"
app:popupTheme="@style/AppTheme"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:minHeight="?attr/actionBarSize"
android:background="?attr/colorPrimary"/>