RainbowEight

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Runtime.InteropServices.WindowsRuntime;
using System.Threading.Tasks;
using Windows.Foundation;
using Windows.Foundation.Collections;
using Windows.UI.Xaml;
using Windows.UI.Xaml.Controls;
using Windows.UI.Xaml.Controls.Primitives;
using Windows.UI.Xaml.Data;
using Windows.UI.Xaml.Input;
using Windows.UI.Xaml.Media;
using Windows.UI.Xaml.Navigation;

// The Blank Page item template is documented at https://go.microsoft.com/fwlink/?LinkId=402352&clcid=0x409

namespace RainbowEight
{
    /// <summary>
    /// An empty page that can be used on its own or navigated to within a Frame.
    /// </summary>
    public sealed partial class MainPage : Page
    {
        double txtblkBaseSize;
        Random rand = new Random();
        public MainPage()
        {
            this.InitializeComponent();
            Loaded += OnPageLoad;
        }

        void OnPageLoad(object sender,RoutedEventArgs e)
        {
            txtblkBaseSize = txtblk.ActualHeight;
            CompositionTarget.Rendering += OnCompositionTargetRendering;
        }

        void OnCompositionTargetRendering(object sender,object e)
        {
            // TimeSpan.FromSeconds(30000)
            //            Task.Delay(30);
 //           Sleep(1000);

//            txtblk.Text = (rand.Next() % 10).ToString();
            
            txtblk.FontSize = ActualHeight / txtblkBaseSize;

            RenderingEventArgs renderingArgs = e as RenderingEventArgs;

            double t = (0.25 * renderingArgs.RenderingTime.TotalSeconds) % 1;

            for (int index=0; index < gradientBrush.GradientStops.Count; index++)
            {
                gradientBrush.GradientStops[index].Offset = index / 7.0 - t;
            }
        }

        static void Sleep(int ms)
        {
            new System.Threading.ManualResetEvent(false).WaitOne(ms);
        }

        private void OnPage_SizeChanged(object sender, SizeChangedEventArgs e)
        {
                        txtblkBaseSize = txtblk.ActualHeight;
            //            
            //  txtblk.FontSize = 8;
            txtblk.FontSize = ActualHeight / txtblkBaseSize;
        }
    }
}

<Page
    x:Class="RainbowEight.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:RainbowEight"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d">

    <Grid Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">
        <TextBlock Name="txtblk"
                   Text="8"
                   FontFamily="CooperBlack"
                   FontSize="1"
                   HorizontalAlignment="Center"
                   >
            <TextBlock.Foreground>
                <LinearGradientBrush x:Name="gradientBrush">
                    <GradientStop Offset="0.00" Color="Red"/>
                    <GradientStop Offset="0.14" Color="Orange"/>
                    <GradientStop Offset="0.28" Color="Yellow"/>
                    <GradientStop Offset="0.43" Color="Green"/>
                    <GradientStop Offset="0.57" Color="Blue"/>
                    <GradientStop Offset="0.71" Color="Indigo"/>
                    <GradientStop Offset="0.86" Color="Violet"/>
                    <GradientStop Offset="1.00" Color="Red"/>
                    <GradientStop Offset="1.14" Color="Orange"/>
                    <GradientStop Offset="1.28" Color="Yellow"/>
                    <GradientStop Offset="1.43" Color="Green"/>
                    <GradientStop Offset="1.57" Color="Blue"/>
                    <GradientStop Offset="1.71" Color="Indigo"/>
                    <GradientStop Offset="1.86" Color="Violet"/>
                    <GradientStop Offset="2.00" Color="Red"/>
                </LinearGradientBrush>
            </TextBlock.Foreground>
        </TextBlock>
    </Grid>
</Page>