Changing ToolbarItem text color via xaml to all platform(Android,iOS,UWP)

If you want to change your toolbar textcolor, you can use Navigation.Titleview

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:local="clr-namespace:TitleViewSample"
             x:Class="TitleViewSample.MainPage">

      <NavigationPage.TitleView>
        <Grid>
            <Grid.ColumnDefinitions>
                <ColumnDefinition Width="*"/>
                <ColumnDefinition Width="Auto"/>
            </Grid.ColumnDefinitions>

            <Label Text="Your Title" Grid.Column="0" TextColor="White" FontSize="Large" VerticalOptions="Center" HorizontalOptions="FillAndExpand"></Label>

            <StackLayout Grid.Column="1" HorizontalOptions="EndAndExpand" Orientation="Horizontal" Margin="0,5,5,5">
                <Label Text="?" FontSize="Large" TextColor="Red" Margin="10" ></Label>
                <Label Text="+" FontSize="Large" TextColor="Green" Margin="10" ></Label>
            </StackLayout>
        </Grid>
</NavigationPage.TitleView>


</ContentPage>

If you want to change the toolbar color add this code when navigating to your specific page

 MainPage = new NavigationPage(new YourPage());
((NavigationPage)MainPage).BarBackgroundColor = Color.FromHex("#19110F");