Android button margins when background color added

I have problem with button margins (Android Studio) when I change background colors of buttons. All buttons are the same size but those added background color has no space between them. How can I fix this?

This code is for buttons for uncolored buttons

<Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/btnAdd"
        android:id="@+id/btnAdd"
        android:layout_above="@+id/btnSub"
        android:layout_centerHorizontal="true"/>

And I added this line for get background color on colored buttons.

android:background="@color/red"

If I delete all background attributes from middle buttons then all buttons looks same same and in sync.