TabLayout color of selected tab underline

Use app:tabIndicatorColor.

Example:

<android.support.design.widget.TabLayout
    android:id="@+id/tabs"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    app:tabIndicatorColor="@android:color/white" />

Make sure you have this namespace: xmlns:app="http://schemas.android.com/apk/res-auto"

Documentation: https://developer.android.com/reference/android/support/design/widget/TabLayout.html#attr_android.support.design:tabIndicatorColor.