Fist declare green colour in your colors.xml file
 <?xml version="1.0" encoding="utf-8"?>
<resources>
    <color name="green">#3cb879</color>
    <color name="white">#ffffff</color>
</resources>
then create a drawable file called green_button.xml and cop this code 
        <?xml version="1.0" encoding="UTF-8"?>
<selector xmlns:android="http://schemas.android.com/apk/res/android">
    <item android:state_pressed="true">
        <shape>
            <solid android:color="#e78181" />
            <stroke android:width="5dip" android:color="@color/green" />
            <corners android:radius="10dip" />
            <padding android:left="0dp" android:top="0dp" android:right="0dp" android:bottom="0dp" />
        </shape>
    </item>
    <item>
        <shape>
            <gradient android:startColor="#ffffff" android:endColor="#ffffff" android:angle="270" />
            <stroke android:width="1dp" android:color="#ffffff" />
            <corners android:radius="4dp" />
            <padding android:left="0dp" android:top="0dp" android:right="0dp" android:bottom="0dp" />
        </shape>
    </item>
</selector>
and then in activity_main.xml create button and apply background to it like this.
<Button
        android:layout_width="150dp"
        android:layout_height="60dp"
        android:text="OPEN"
        android:textColor="@color/green"
        android:textSize="30sp"
        android:layout_margin="5dp"
        android:background="@drawable/green_button"/>
see output here