Change toolbar back arrow color

You can override the theme in your Toolbar.
With a Material Components theme:

  <com.google.android.material.appbar.MaterialToolbar
        style="@style/Widget.MaterialComponents.Toolbar.Primary"
        android:theme="@style/MyThemeOverlay_Toolbar"
        ..>

with:

  <style name="MyThemeOverlay_Toolbar2" parent="ThemeOverlay.MaterialComponents.Toolbar.Primary">

    <!-- This attributes is used by navigation icon and overflow icon -->
    <item name="colorOnPrimary">@color/secondaryColor</item>
  </style>

With an AppCompat theme:

<android.support.v7.widget.Toolbar
  android:theme="@style/myToolbarTheme" 
  ...
>

Then in your theme you can define the colorControlNormal attribute:

   <style name=""  parent="ThemeOverlay.AppCompat.Dark.ActionBar">
      ....
      <item name="colorControlNormal">@color/myColor</item>  
   </style>

Be mindful of the theme you use. If you copied your ToolBar code from https://developer.android.com/training/appbar/setting-up, then you have this:

<android.support.v7.widget.Toolbar
   android:id="@+id/my_toolbar"
   android:layout_width="match_parent"
   android:layout_height="?attr/actionBarSize"
   android:background="?attr/colorPrimary"
   android:elevation="4dp"
   android:theme="@style/ThemeOverlay.AppCompat.ActionBar"
   app:popupTheme="@style/ThemeOverlay.AppCompat.Light"/>

Based on @Gabriele's answer (upvoted), I had to take out the android:theme="@style/ThemeOverlay.AppCompat.ActionBar" attribute and put it in styles.xml then customize the colorControlNormal attribute like so:

<style name="ToolBarTheme"  parent="ThemeOverlay.AppCompat.ActionBar">
    <item name="colorControlNormal">@color/white</item>
</style>

Back to my Toolbar declaration, I modified as below:

<android.support.v7.widget.Toolbar
   ...
   android:theme="@style/ToolBarTheme"
   />

Cheers!