At First

Caution: This article is sensitive because it may depends on Android SDK Version or design support library com.android.support:design version.

Confusing from many solutions on web.

I searched many article by google but cannot resolved.
Finally I got a solution.

Preconditions

I tried to set to app/build.gradle file like this.

apply plugin: 'com.android.application'

apply plugin: 'kotlin-android'

apply plugin: 'kotlin-android-extensions'

android {
    compileSdkVersion 28
    defaultConfig {
        applicationId "com.example.actionbartextcolor"
        minSdkVersion 26
        targetSdkVersion 28
        versionCode 1
        versionName "1.0"
        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
        }
    }
}

dependencies {
    implementation fileTree(dir: 'libs', include: ['*.jar'])
    implementation"org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"
    implementation 'com.android.support:appcompat-v7:28.0.0'
    implementation 'com.android.support:design:28.0.0'
    implementation 'com.android.support.constraint:constraint-layout:1.1.3'
}

The Nex Sample cannot apply the custom color to ActionBar in SDK Lv28. this way less than SDK Lv28.

<resources>

    <!-- Base application theme. -->
    <style name="AppTheme" parent="Theme.MaterialComponents.Light.DarkActionBar">
        <!-- Customize your theme here. -->
        <item name="colorPrimary">@color/colorPrimary</item>
        <item name="colorPrimaryDark">@color/colorPrimaryDark</item>
        <item name="colorAccent">@color/colorAccent</item>
        <item name="actionBarStyle">@style/MyActionBar</item>
    </style>

   <!-- this cannot apply textColor -->

    <style name="MyActionBar"
        parent="@style/Widget.AppCompat.ActionBar">
        <item name="android:titleTextStyle">@style/TitleTextStyle</item>
        <item name="android:background">@color/colorPrimary</item>
        <item name="android:backgroundStacked">@color/colorPrimary</item>
        <item name="android:backgroundSplit">@color/colorPrimary</item>

        <!-- Support library compatibility -->
        <item name="titleTextStyle">@style/TitleTextStyle</item>
        <item name="background">@color/colorPrimary</item>
        <item name="backgroundStacked">@color/colorPrimary</item>
        <item name="backgroundSplit">@color/colorPrimary</item>
    </style>

    <style name="TitleTextStyle"
        parent="@style/TextAppearance.AppCompat.Widget.ActionBar.Title">
        <item name="android:textColor">@color/colorAccent</item>
    </style>
</resources>

Right way is Next.

<resources>

    <!-- Base application theme. -->
    <style name="AppTheme" parent="Theme.MaterialComponents.Light.DarkActionBar">
        <!-- Customize your theme here. -->
        <item name="colorPrimary">@color/colorPrimary</item>
        <item name="colorPrimaryDark">@color/colorPrimaryDark</item>
        <item name="colorAccent">@color/colorAccent</item>
        <item name="toolbarStyle">@style/CustomToolBarStyle</item>
    </style>

    <!-- ActionBar is no longer Toolbar -->
    
    <style name="CustomToolBarStyle" parent="@style/Widget.AppCompat.Toolbar">
        <item name="titleTextColor">@color/colorAccent</item>
    </style>

</resources>

How did I get it?

I got ActionBar Instance's Class with Debugger.
And I researched who have title text.
The Answer is android.support.v7.widget.Toolbar.
Finally I found the Next code.

        if (a.hasValue(styleable.Toolbar_titleTextColor)) {
            this.setTitleTextColor(a.getColor(styleable.Toolbar_titleTextColor, -1));
        }

The teachings from attaching style

Do not use google.
Just run.