Android :: Android - Color State List Resources - Specify A Background Color

To give the user of my app an indication which field currently has the focus I am trying to change the background color of some of my fields depending on the current state, however, I am having troubles understanding Androids Color State List Resources:

There is this example at the bottom of http://developer.android.com/guide/topics/resources/color-list-resource.html. If I try exactly the same, i.e. if I want to adapt the textColor , things do work. However, if I try an only slightly different thing, namely to adapt the background color, things do not work and I don't understand why? Why is this so inconsistent?

To make it simpler to understand what I am trying to do, I append my misc. .xml files:

The AndroidManifest.xml file:

CODE:.........

If I run this as shown here, it works, i.e. I get a button whose text color changes depending on whether the button is focuses, pressed, etc.

If I uncomment the lower button, where I just flipped the attribute values for textColor and background I get an exception, stating

... <item> tag requires a 'drawable' attribute or child tag defining a drawable

What the heck am I missing here? Why is that color state list acceptable as a text color but not as a background color? How does one specify a view's background color depending on the view's state?