Change ActionBar title text color using Light.DarkActionBar theme in AppCompat 21

You can change it with actionBarStyle attribute of theme.

<style name="AppBaseTheme" parent="@style/Theme.AppCompat.Light.DarkActionBar">
    <item name="actionBarStyle">@style/MyActionBar</item>
</style>

<style name="MyActionBar" parent="@style/Widget.AppCompat.ActionBar.Solid">
    <item name="titleTextStyle">@style/MyTitleTextStyle</item>
</style>

<style name="MyTitleTextStyle" parent="@style/TextAppearance.AppCompat.Widget.ActionBar.Title">
    <item name="android:textColor">CHANGE_COLOR_HERE</item>
</style>

This will make your Toolbar Title use white color:

<android.support.v7.widget.Toolbar
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:id="@+id/toolbar"
    android:layout_height="wrap_content"
    android:layout_width="match_parent"
    app:theme="@style/ThemeOverlay.AppCompat.Dark.ActionBar"
    android:minHeight="?attr/actionBarSize"
    android:background="?attr/colorPrimary" />

To understand what is the difference between ThemeOverlay.AppCompat.Dark.ActionBar and Theme.AppCompat.Light.DarkActionBar check this answer

It's app:theme="@style/ThemeOverlay.AppCompat.Dark.ActionBar" that applies a light colour because the title colour which is used in this theme is white.