Selected tab's color in Bottom Navigation View

While making a selector, always keep the default state at the end, otherwise only default state would be used. You need to reorder the items in your selector as:

<?xml version="1.0" encoding="utf-8"?>
<selector xmlns:android="http://schemas.android.com/apk/res/android">
    <item android:state_checked="true" android:color="@android:color/holo_blue_dark" />
    <item android:color="@android:color/darker_gray"  />
</selector>

And the state to be used with BottomNavigationBar is state_checked not state_selected.

1. Inside res create folder with name color (like drawable)

2. Right click on color folder. Select new-> color resource file-> create color.xml file (bnv_tab_item_foreground) (Figure 1: File Structure)

3. Copy and paste bnv_tab_item_foreground

<android.support.design.widget.BottomNavigationView
            android:id="@+id/navigation"
            android:layout_width="0dp"
            android:layout_height="wrap_content"
            android:layout_marginEnd="0dp"
            android:layout_marginStart="0dp"
            app:itemBackground="@color/appcolor"//diffrent color
            app:itemIconTint="@color/bnv_tab_item_foreground" //inside folder 2 diff colors
            app:itemTextColor="@color/bnv_tab_item_foreground"
            app:layout_constraintBottom_toBottomOf="parent"
            app:layout_constraintLeft_toLeftOf="parent"
            app:layout_constraintRight_toRightOf="parent"
            app:menu="@menu/navigation" />

bnv_tab_item_foreground:

 <?xml version="1.0" encoding="utf-8"?>
    <selector xmlns:android="http://schemas.android.com/apk/res/android">
        <item android:state_checked="true" android:color="@color/white" />
        <item android:color="@android:color/darker_gray"  />
    </selector>

Figure 1: File Structure:

BottomNavigationView uses colorPrimary from the theme applied for the selected tab and it uses android:textColorSecondary for the inactive tab icon tint.

So you can create a style with the prefered primary color and set it as a theme to your BottomNavigationView in an xml layout file.

styles.xml:

 <style name="BottomNavigationTheme" parent="Theme.AppCompat.Light">
        <item name="colorPrimary">@color/active_tab_color</item>
        <item name="android:textColorSecondary">@color/inactive_tab_color</item>
 </style>

your_layout.xml:

<android.support.design.widget.BottomNavigationView
            android:id="@+id/navigation"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:background="?android:attr/windowBackground"
            android:theme="@style/BottomNavigationTheme"
            app:menu="@menu/navigation" />