Sorry for the delay in sending this.

I created a simple Crosslight (Android) project which emulate the Email validator behavior. Following steps shows the detail:

1. open Visual Studio and select New, Project.
2. From the dialog that appears, choose Visual C#, Intersoft Solutions, Mobile, then select Intersoft Crosslight Application (iOS, Android, Windows). Enter other necessary information such as Name, Location and Solution Name.
   
3. Create a Crosslight Blank solution, by choosing Blank from the drop down and hit OK.
   
4. Once the project is created, open SimpleViewModel.cs located inside the [Project Name].Core project.
5. Locate NewText property inside the SimpleViewModel class and replace the entire code as follows:
   

   public string NewText
   {
       get { return _newText; }
       set
       {
           if (_newText != value)
           {
               _newText = value;
   
               const string emailRegex = @"^(?("")("".+?(?<!\\)""@)|(([0-9a-z]((\.(?!\.))|[-!#\$%&'\*\+/=\?\^`\{\}\|~\w])*)(?<=[0-9a-z])@))" +
                   @"(?(\[)(\[(\d{1,3}\.){3}\d{1,3}\])|(([0-9a-z][-\w]*[0-9a-z]*\.)+[a-z0-9][\-a-z0-9]{0,22}[a-z0-9]))$";
   
               var IsValid = (Regex.IsMatch(_newText, emailRegex, RegexOptions.IgnoreCase, TimeSpan.FromMilliseconds(250)));
   
               if (IsValid)
                   GreetingText = "Your email looks good";
               else
                   GreetingText = "Enter a valid email";
   
               OnPropertyChanged("NewText");
           }
       }
   }

   
   In the setter of NewText, email validation is added. The email validation uses emailRegex string constant and IsMatch method. When IsValid expression returns true, GreetingText property will be set to "Your email looks good". Else, it will be set set to "Enter a valid email".
6. Add a folder in the [Project Name].Core project and named it as Converters.
7. Add a class and named it as TextStyleConverter.cs into the Converters folder.
8. Copy and replace the entire code of TextStyleConverter class as follows:
   

   using Intersoft.Crosslight;
   using Intersoft.Crosslight.Drawing;
   using System;
   using System.Globalization;
   
   namespace EmailValidator.Converters
   {
       public class TextStyleConverter : IValueConverter
       {
           #region IValueConverter implementation
   
           public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
           {
               if (value != null)
               {
                   var styles = new StyleAttributes();
   
                   if (value.ToString() == "Your email looks good")
                       styles.ForegroundColor = Colors.Green;
                   else
                       styles.ForegroundColor = Colors.Red;
   
                   return styles;
               }
               return value;
           }
   
           public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
           {
               throw new NotSupportedException();
           }
   
           #endregion
       }
   }

   
   This converter sets ForegroundColor to green when email is valid and sets ForegroundColor to red when email is not valid.
9. Open SimpleBindingProvider.cs located inside the [ProjectName].Core project.
10. Add following snippet code into the constructor of SimpleBindingProvider class.
    

    this.AddBinding("GreetingLabel", BindableProperties.StyleAttributesProperty, new BindingDescription("GreetingText")
    {
        Converter = new TextStyleConverter()
    });

    
    The code adds a binding provider which acts as the mediator that provides binding definitions to the interested parties such as view models and views. The GreetingLabel is the view target contract; the BindableProperties.StyleAttributesProperty is the target property to bind; and the rest is the binding description to the converter.
11. Save the changes and run the project.

I enclosed the simple Android project (as attachment) for your reference. Hope this helps.