ImageButton button - pressed grey.

<ImageButton
  android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:background="@drawable/mainbt_selector"
    android:scaleType="centerInside"
    android:src="@drawable/add"/>

<?xml version="1.0" encoding="utf-8"?>
<selector xmlns:android="http://schemas.android.com/apk/res/android" >
     <item android:drawable="@drawable/mainbt_pressed"   android:state_pressed="true"/>
    <item android:drawable="@drawable/mainbt" />

</selector>

mainbt.xml

<?xml version="1.0" encoding="utf-8"?>
<shape xmlns:android="http://schemas.android.com/apk/res/android" >
    <solid
       android:color="#C8C8C8"/>
    <corners
        android:radius="20dp"/>

</shape>

main_pressed.xml

<?xml version="1.0" encoding="utf-8"?>
<shape xmlns:android="http://schemas.android.com/apk/res/android" >
    <solid
       android:color="#C8C8C8"/>
    <corners
        android:radius="20dp"/>

</shape>