Zadanie

<StackLayout>
   
    
    
<StackLayout WidthRequest="900" HeightRequest="900">

    <StackLayout x:Name="jola" 
                 Orientation="Horizontal"
                 WidthRequest="800"
                 HeightRequest="150">
        <Label Text="Jola" FontSize="40" Margin="0,30"/>
        <StackLayout Margin="150,0">
            <Label Text ="INF.03" FontSize="30" HorizontalTextAlignment="Center"/>
            <Entry x:Name="inf03Jola" WidthRequest="200"
                   HorizontalTextAlignment="Center"/>

        </StackLayout>
        <StackLayout>
            <Label Text ="INF.04" FontSize="30" HorizontalTextAlignment="Center"/>
            <Entry x:Name="inf04Jola" WidthRequest="200"
                   HorizontalTextAlignment="Center"/>

        </StackLayout>

    </StackLayout>

    <StackLayout x:Name="Edek" 
     Orientation="Horizontal"
     WidthRequest="800"
     HeightRequest="150">
        <Label Text="Edek" FontSize="40" Margin="0,30"/>
        <StackLayout Margin="110,0">
            <Label Text ="INF.03" FontSize="30" HorizontalTextAlignment="Center"/>
            <Entry x:Name="inf03Edek" WidthRequest="200" MaxLength="1"
       HorizontalTextAlignment="Center" TextChanged="funkcja1">

                <Entry.Triggers>
                    <DataTrigger TargetType="Entry"
                                 Binding="{Binding Source={x:Reference inf03Edek},
                                 Path=Text}"
                                 Value="0" >
                        <Setter Property="BackgroundColor"
                            Value="Red"/>
                        <Setter Property="TextColor"
                                Value="White"/>

                    </DataTrigger>
                    <DataTrigger TargetType="Entry"
                                 Binding="{Binding Source={x:Reference inf03Edek},
                                 Path=Text}"
                                 Value="1" >
                        <Setter Property="BackgroundColor"
                                     Value="Green"/>
                        <Setter Property="TextColor"
                    Value="White"/>

                    </DataTrigger>
                </Entry.Triggers>
            </Entry>
                <Label x:Name="komunikat" Text="Musisz wskazać liczbę 0 lub 1 !"/>
            </StackLayout>
        <StackLayout>
            <Label Text ="INF.04" FontSize="30" HorizontalTextAlignment="Center"/>
            <Entry x:Name="inf04Edek" WidthRequest="200"
       HorizontalTextAlignment="Center"/>

        </StackLayout>

    </StackLayout>
    <StackLayout x:Name="filemon" 
     Orientation="Horizontal"
     WidthRequest="800"
     HeightRequest="150">
        <Label Text="Filemon" FontSize="40" Margin="0,30"/>
        <StackLayout Margin="50,0">
            <Label Text ="INF.03" FontSize="30" HorizontalTextAlignment="Center"/>
            <Entry x:Name="inf03filemon" WidthRequest="200"
       HorizontalTextAlignment="Center"/>

        </StackLayout>
        <StackLayout>
            <Label Text ="INF.04" FontSize="30" HorizontalTextAlignment="Center"/>
            <Entry x:Name="inf04filemon" WidthRequest="200"
       HorizontalTextAlignment="Center"/>

        </StackLayout>

    </StackLayout>


</StackLayout>
</StackLayout>

public MainPage()
{
InitializeComponent();
komunikat.IsVisible = false;
}

private void funkcja1(object sender, TextChangedEventArgs e)
{
var obiekt = sender as Entry;
// ta wersja jest niebezpieczna bo
// w przypadku tekstu : abc wyrzuci wyjatek lepiej dać
// Try parse
//double wartosc = Convert.ToDouble(obiekt.Text);

 if ((double.TryParse(obiekt.Text, out double wartosc)))
 {
     if ((wartosc >= 0) && (wartosc <= 1))
     {
         komunikat.IsVisible = false;

     }
     else
     {


         obiekt.BackgroundColor = Colors.Yellow;
         komunikat.IsVisible = true;

     }
 }

}