I was having this same issue and while looking at one of the platform drawable XML files I noticed a way to eliminate the need to create an image file just for a color, by creating a shape in XML.
For example, instead of:
<item android:state_focused="true"
android:drawable="@drawable/list_active" />
Do:
<item android:state_focused="true">
<shape>
<solid android:color="#66000000" />
</shape>
</item>
Beyond just creating a plain color drawable, the shape is flexible, similar to a simple vector object. All the details can be found here: http://developer.android.com/guide/topics/resources/drawable-resource.html#Shape