How to set icon color of MenuItem?

The icon is actually provided by the ShareActionProvider and you can’t change it afaik. You can, however, customize the color by setting the textColorPrimary in your styles.xml:

<android.support.v7.widget.Toolbar
    android:id="@+id/toolbar"
    android:layout_width="match_parent"
    android:layout_height="?attr/actionBarSize"
    app:theme="@style/MyActionBarTheme"
    app:popupTheme="@style/ThemeOverlay.AppCompat.Light">

<style name="MyActionBarTheme" parent="ThemeOverlay.AppCompat.Dark.ActionBar">
    <item name="android:textColorPrimary">#fa0</item>
</style>

For any custom icons, you would have to color them yourself, ie.

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    getMenuInflater().inflate(R.menu.menu_main, menu);

    for(int i = 0; i < menu.size(); i++){
        Drawable drawable = menu.getItem(i).getIcon();
        if(drawable != null) {
            drawable.mutate();
            drawable.setColorFilter(getResources().getColor(R.color.textColorPrimary), PorterDuff.Mode.SRC_ATOP);
        }
    }

    return true;
}