The problem here is that you cannot define the background color using a color selector, you need a drawable selector. So, the necessary changes would look like this:
<?xml version="1.0" encoding="utf-8"?>
<selector xmlns:android="http://schemas.android.com/apk/res/android">
<item
android:state_pressed="true"
android:drawable="@drawable/selected_state" />
</selector>
You would also need to move that resource to the drawable directory where it would make more sense since it’s not a color selector per se.
Then you would have to create the res/drawable/selected_state.xml file like this:
<?xml version="1.0" encoding="UTF-8"?>
<shape xmlns:android="http://schemas.android.com/apk/res/android"
android:shape="rectangle">
<solid android:color="@color/semitransparent_white" />
</shape>
and finally, you would use it like this:
android:background="@drawable/selector"
Note: the reason why the OP was getting an image resource drawn is probably because he tried to just reference his resource that was still in the color directory but using @drawable so he ended up with an ID collision, selecting the wrong resource.
Hope this can still help someone even if the OP probably has, I hope, solved his problem by now.