As your error states, you have to define drawable attibute for the items (for some reason it is required when it comes to background definitions), so:
<?xml version="1.0" encoding="utf-8"?>
<selector xmlns:android="http://schemas.android.com/apk/res/android">
<item android:state_pressed="true" android:drawable="@color/red"/> <!-- pressed -->
<item android:state_focused="true" android:drawable="@color/blue"/> <!-- focused -->
<item android:drawable="@color/black"/> <!-- default -->
</selector>
Also note that drawable attribute doesn't accept raw color values, so you have to define the colors as resources. Create colors.xml file at res/values folder:
<?xml version="1.0" encoding="utf-8"?>
<resources>
<color name="black">#000</color>
<color name="blue">#00f</color>
<color name="red">#f00</color>
</resources>