Кнопки на черном фоне с цветами радуги

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity"
    android:background="@color/black">

    <Button
        android:id="@+id/btn_green"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"

        android:layout_centerInParent="true"

        android:textAllCaps="true"
        android:textColor="@color/black"
        android:backgroundTint="@color/green"
        android:text="Green"
        />

    <Button
        android:id="@+id/btn_yellow"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"

        android:layout_above="@+id/btn_green"
        android:layout_alignLeft="@id/btn_green"
        android:layout_marginBottom="-11dp"

        android:backgroundTint="@color/yellow"
        android:text="Yellow"
        android:textAllCaps="true"
        android:textColor="@color/black"
        />

    <Button
        android:id="@+id/btn_orange"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"

        android:layout_above="@+id/btn_yellow"
        android:layout_alignLeft="@id/btn_green"
        android:layout_marginBottom="-11dp"

        android:backgroundTint="@color/orange"
        android:text="Orange"
        android:textAllCaps="true"
        android:textColor="@color/black"
        />

    <Button
        android:id="@+id/btn_red"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"

        android:layout_above="@+id/btn_orange"
        android:layout_alignLeft="@id/btn_green"
        android:layout_marginBottom="-11dp"

        android:backgroundTint="@color/red"
        android:text="Red"
        android:textAllCaps="true"
        android:textColor="@color/black"
        />

    <Button
        android:id="@+id/btn_blue"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"

        android:layout_below="@+id/btn_green"
        android:layout_alignLeft="@id/btn_green"
        android:layout_marginTop="-11dp"

        android:backgroundTint="@color/blue"
        android:text="Blue"
        android:textAllCaps="true"
        android:textColor="@color/white"
        />

    <Button
        android:id="@+id/btn_indigo"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"

        android:layout_below="@+id/btn_blue"
        android:layout_alignLeft="@id/btn_green"
        android:layout_marginTop="-11dp"

        android:backgroundTint="@color/indigo"
        android:text="Indigo"
        android:textAllCaps="true"
        android:textColor="@color/white"
        />

    <Button
        android:id="@+id/btn_violet"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"

        android:layout_below="@+id/btn_indigo"
        android:layout_alignLeft="@id/btn_green"
        android:layout_marginTop="-11dp"

        android:backgroundTint="@color/violet"
        android:text="Violet"
        android:textAllCaps="true"
        android:textColor="@color/black"
        />

</RelativeLayout>

colors.xml

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <color name="purple_200">#FFBB86FC</color>
    <color name="purple_500">#FF6200EE</color>
    <color name="purple_700">#FF3700B3</color>
    <color name="teal_200">#FF03DAC5</color>
    <color name="teal_700">#FF018786</color>
    <color name="black">#FF000000</color>
    <color name="white">#FFFFFFFF</color>

    <!--Rainbow colors-->
    <color name="red">#FF1E1E</color>
    <color name="orange">#FF871E</color>
    <color name="yellow">#FFE91E</color>
    <color name="green">#25FF1E</color>
    <color name="blue">#0942D3</color>
    <color name="indigo">#5B02AA</color>
    <color name="violet">#E06FFF</color>
</resources>