Blue
<?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="#449def" />
<stroke
android:width="1dp"
android:color="#2f6699" />
<corners
android:radius="3dp" />
<padding
android:left="10dp"
android:top="10dp"
android:right="10dp"
android:bottom="10dp" />
</shape>
</item>
<item>
<shape>
<gradient
android:startColor="#449def"
android:endColor="#2f6699"
android:angle="270" />
<stroke
android:width="1dp"
android:color="#2f6699" />
<corners
android:radius="4dp" />
<padding
android:left="10dp"
android:top="10dp"
android:right="10dp"
android:bottom="10dp" />
</shape>
</item>
</selector>
red
<?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="#ef4444" />
<stroke
android:width="1dp"
android:color="#992f2f" />
<corners
android:radius="3dp" />
<padding
android:left="10dp"
android:top="10dp"
android:right="10dp"
android:bottom="10dp" />
</shape>
</item>
<item>
<shape>
<gradient
android:startColor="#ef4444"
android:endColor="#992f2f"
android:angle="270" />
<stroke
android:width="1dp"
android:color="#992f2f" />
<corners
android:radius="3dp" />
<padding
android:left="10dp"
android:top="10dp"
android:right="10dp"
android:bottom="10dp" />
</shape>
</item>
</selector>
purple
<?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="#a276eb" />
<stroke
android:width="1dp"
android:color="#6a3ab2" />
<corners
android:radius="3dp" />
<padding
android:left="10dp"
android:top="10dp"
android:right="10dp"
android:bottom="10dp" />
</shape>
</item>
<item>
<shape>
<gradient
android:startColor="#a276eb"
android:endColor="#6a3ab2"
android:angle="270" />
<stroke
android:width="1dp"
android:color="#6a3ab2" />
<corners
android:radius="4dp" />
<padding
android:left="10dp"
android:top="10dp"
android:right="10dp"
android:bottom="10dp" />
</shape>
</item>
</selector>
green
<?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="#70c656" />
<stroke
android:width="1dp"
android:color="#53933f" />
<corners
android:radius="3dp" />
<padding
android:left="10dp"
android:top="10dp"
android:right="10dp"
android:bottom="10dp" />
</shape>
</item>
<item>
<shape>
<gradient
android:startColor="#70c656"
android:endColor="#53933f"
android:angle="270" />
<stroke
android:width="1dp"
android:color="#53933f" />
<corners
android:radius="4dp" />
<padding
android:left="10dp"
android:top="10dp"
android:right="10dp"
android:bottom="10dp" />
</shape>
</item>
</selector>
yellow
<?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="#f3ae1b" />
<stroke
android:width="1dp"
android:color="#bb6008" />
<corners
android:radius="3dp" />
<padding
android:left="10dp"
android:top="10dp"
android:right="10dp"
android:bottom="10dp" />
</shape>
</item>
<item>
<shape>
<gradient
android:startColor="#f3ae1b"
android:endColor="#bb6008"
android:angle="270" />
<stroke
android:width="1dp"
android:color="#bb6008" />
<corners
android:radius="4dp" />
<padding
android:left="10dp"
android:top="10dp"
android:right="10dp"
android:bottom="10dp" />
</shape>
</item>
</selector>
black
<?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="#343434" />
<stroke
android:width="1dp"
android:color="#171717" />
<corners
android:radius="3dp" />
<padding
android:left="10dp"
android:top="10dp"
android:right="10dp"
android:bottom="10dp" />
</shape>
</item>
<item>
<shape>
<gradient
android:startColor="#343434"
android:endColor="#171717"
android:angle="270" />
<stroke
android:width="1dp"
android:color="#171717" />
<corners
android:radius="4dp" />
<padding
android:left="10dp"
android:top="10dp"
android:right="10dp"
android:bottom="10dp" />
</shape>
</item>
</selector>
style
<style name="ButtonText">
<item name="android:layout_width">fill_parent</item>
<item name="android:layout_height">wrap_content</item>
<item name="android:textColor">#ffffff</item>
<item name="android:gravity">center</item>
<item name="android:layout_margin">3dp</item>
<item name="android:textSize">30dp</item>
<item name="android:textStyle">bold</item>
<item name="android:shadowColor">#000000</item>
<item name="android:shadowDx">1</item>
<item name="android:shadowDy">1</item>
<item name="android:shadowRadius">2</item>
</style>
sample layout
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" android:layout_width="fill_parent" android:layout_height="fill_parent"
android:orientation="vertical">
<Button android:text="Button" android:id="@+id/button1" android:background="@drawable/btn_red" style="@style/ButtonText"></Button>
<Button android:text="Button" android:id="@+id/button2" android:background="@drawable/btn_blue" style="@style/ButtonText"></Button>
<Button android:text="Button" android:id="@+id/button3" android:background="@drawable/btn_purple" style="@style/ButtonText"></Button>
<Button android:text="Button" android:id="@+id/button4" android:background="@drawable/btn_green" style="@style/ButtonText"></Button>
<Button android:text="Button" android:id="@+id/button5" android:background="@drawable/btn_orange" style="@style/ButtonText"></Button>
<Button android:text="Button" android:id="@+id/button6" android:background="@drawable/btn_black" style="@style/ButtonText"></Button>
</LinearLayout>
http://www.dibbus.com/2012/08/more-buttons/
https://code.google.com/p/android-gradients-sample/