Yes, you can change the focus color of a button (this an open source OS). I poked around the SDK and could not find a direct way to change the focus color of a button. I did find this tantalizing xml file...
widget_button.xml
<selector xmlns:android="http://schemas.android.com/apk/res/android">
<item android:state_focused="true" android:color="#ff000000"/>
<item android:color="#ff000000"/> <!-- unfocused -->
</selector>
...but I'm having a hard time figuring out where to adjust things. Button extends TextView, and based on the TextView source that's where the focus color is defined.
If you really want to this you need to make your own class from source.
PS: Since this is so hard I suspect it's discouraged by the Andoird team. It makes for a less consistent user interface.