change color of only ONE menu item icon in android

UPDATE1 : Adding a way to avoid xml and code duplication.

UPDATE2 : Adding static listener.

I don't know if you can achieve your goal just by changing your menu.xml but the Toolbar is a viewGroup so you can achieve your goal with a view.xml which should looks like it :

toolbar.xml :

    <LinearLayout
        android:layout_width="?attr/actionBarSize"
        android:layout_height="match_parent"
        android:layout_marginRight="8dp"
        android:background="#000000">

        <Button
            android:id="@+id/button_first"
            android:layout_width="?attr/actionBarSize"
            android:layout_height="match_parent"
            android:background="@android:drawable/ic_lock_idle_lock" />

    </LinearLayout>

    <LinearLayout
        android:layout_width="?attr/actionBarSize"
        android:layout_height="match_parent"
        android:layout_marginRight="8dp"
        android:background="#000000">

        <Button
            android:id="@+id/button_second"
            android:layout_width="?attr/actionBarSize"
            android:layout_height="match_parent"
            android:background="@android:drawable/ic_lock_idle_lock" />

    </LinearLayout>

</android.support.v7.widget.Toolbar>

It can be boring to add this code on all of your xml so i advice you to to use thé <include /> attribute like this :

main.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
                android:layout_width="match_parent"
                android:layout_height="match_parent">

    <include
        android:id="@+id/toolbar"
        layout="@layout/toolbar"/>


</RelativeLayout>

Then, you just have to set the Toolbar and use the Button

MainActivity.java

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);

    final Button buttonFirst = (Button) findViewById(R.id.button_first);
    final Button buttonSecond = (Button) findViewById(R.id.button_second);

    buttonFirst.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {

        }
    });

    buttonSecond.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {

        }
   });
}

If all of your button have the same listener, you can make them static into avoid the code duplication :

IconListener.java :

public class IconListener {

    public static View.OnClickListener listenerButtonOne = new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            //DO what you want
        }
    };

    public static View.OnClickListener listenerButtonTwo = new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            //Do what you want
        }
    };
}

So your MainActivity.java became

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);

    final Button buttonFirst = (Button) findViewById(R.id.button_first);
    final Button buttonSecond = (Button) findViewById(R.id.button_second);

    buttonFirst.setOnClickListener(IconListener.listenerButtonOne)
    buttonSecond.setOnClickListener(IconListener.listenerButtonTwo);
}