３つのスライダーを調整して、短形の色を変更します。

<phone:PhoneApplicationPage 
	x:Class="BaseApp.MainPage"
	xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
	xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
	xmlns:phone="clr-namespace:Microsoft.Phone.Controls;assembly=Microsoft.Phone"
	xmlns:shell="clr-namespace:Microsoft.Phone.Shell;assembly=Microsoft.Phone"
	xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
	xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
	mc:Ignorable="d" d:DesignWidth="480" d:DesignHeight="768"
	FontFamily="{StaticResource PhoneFontFamilyNormal}"
	FontSize="{StaticResource PhoneFontSizeNormal}"
	Foreground="{StaticResource PhoneForegroundBrush}"
	SupportedOrientations="Portrait" Orientation="Portrait"
	shell:SystemTray.IsVisible="True">
 
	<!--LayoutRoot is the root grid where all page content is placed-->
	<Canvas x:Name="LayoutRoot" Background="Black"></Canvas>
</phone:PhoneApplicationPage>

using System;
using System.Collections.Generic;
using System.Linq;
using System.Net;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Animation;
using System.Windows.Media.Imaging;
using System.Windows.Shapes;
using Microsoft.Phone.Controls;
using System.Diagnostics;
namespace BaseApp
{
	// エントリーポイント
	public partial class MainPage : PhoneApplicationPage
	{
		private Rectangle rect0 = null;
		private Slider sliderR = null;
		private Slider sliderG = null;
		private Slider sliderB = null;
 
		SolidColorBrush solidColor = null;
 
		// コンストラクタ
		public MainPage()
		{
			// コンポーネントの初期化。
			InitializeComponent();
 
			// 初期化完了後に呼ばれるメソッドの登録。
			Loaded += OnLoaded;
		}
 
		// 初期化完了後に呼ばれるメソッド。
		void OnLoaded(object sender, RoutedEventArgs args)
		{
			// スライダーを表示する。
			{
				sliderR = new Slider();
				sliderR.Name = "sliderR";
				sliderR.Tag = "red";
				sliderR.Width = 255;
				sliderR.Height = 80;
				sliderR.Maximum = 255;
				sliderR.Minimum = 0;
				sliderR.Value = 255;
				sliderR.ValueChanged += new RoutedPropertyChangedEventHandler<double>(OnValueChanged);
				Canvas.SetLeft(sliderR, 20);
				Canvas.SetTop(sliderR, 40);
				LayoutRoot.Children.Add(sliderR);
 
				sliderG = new Slider();
				sliderG.Name = "sliderG";
				sliderG.Tag = "green";
				sliderG.Width = 255;
				sliderG.Height = 80;
				sliderG.Maximum = 255;
				sliderG.Minimum = 0;
				sliderG.Value = 255;
				sliderG.ValueChanged += new RoutedPropertyChangedEventHandler<double>(OnValueChanged);
				Canvas.SetLeft(sliderG, 20);
				Canvas.SetTop(sliderG, 120);
				LayoutRoot.Children.Add(sliderG);
 
				sliderB = new Slider();
				sliderB.Name = "sliderB";
				sliderB.Tag = "blue";
				sliderB.Width = 255;
				sliderB.Height = 80;
				sliderB.Maximum = 255;
				sliderB.Minimum = 0;
				sliderB.Value = 255;
				sliderB.ValueChanged += new RoutedPropertyChangedEventHandler<double>(OnValueChanged);
				Canvas.SetLeft(sliderB, 20);
				Canvas.SetTop(sliderB, 200);
				LayoutRoot.Children.Add(sliderB);
			}
 
			// 短形を描く。
			{
				rect0 = new Rectangle();
				solidColor = new SolidColorBrush(Color.FromArgb(255, (byte)sliderR.Value, (byte)sliderG.Value, (byte)sliderB.Value));
				rect0.Fill = solidColor;
				rect0.Width = 240;
				rect0.Height = 240;
				Canvas.SetLeft(rect0, 120);
				Canvas.SetTop(rect0, 300);
				LayoutRoot.Children.Add(rect0);
			}
		}
 
		// スライダーの値が変更された時に呼ばれるメソッド。
		private void OnValueChanged( object sender, RoutedPropertyChangedEventArgs<double> e )
		{
			Slider sl = sender as Slider;
			String tag = (String)(sl).Tag;
 
			double value = (double)e.NewValue;
			Color color0 = solidColor.Color;
 
			switch (tag)
			{
			case "red":
				solidColor.Color = Color.FromArgb(color0.A, (byte)value, color0.G, color0.B);
				break;
			case "green":
				solidColor.Color = Color.FromArgb(color0.A, color0.R, (byte)value, color0.B);
				break;
			case "blue":
				solidColor.Color = Color.FromArgb(color0.A, color0.R, color0.G, (byte)value);
				break;
			}
 
			rect0.Fill = solidColor;
		}
	}
}