WPF C#

  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
  xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
  xmlns:d="http://schemas.microsoft.com/expression/interactivedesigner/2006"
  mc:Ignorable="d"
  x:Class="PaintDrawExamples.ChangeFill" 
  Width="640" Height="480">
  
    
  

  
    
      
    
  

  
    
      
        
        
        
        
        
      

    

    
      
        
        
      
    

  

//File:Window.xaml.cs
using System;
using System.IO;
using System.Net;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Animation;
using System.Windows.Navigation;
namespace PaintDrawExamples
{
  public partial class ChangeFill
  {
    public ChangeFill()
    {
      this.InitializeComponent();
      this.MouseDown += new MouseButtonEventHandler(MouseDownHandler);
    }
    private void MouseDownHandler(object sender, MouseButtonEventArgs e)
    {
      LinearGradientBrush brush = (LinearGradientBrush)this.myShape.Fill;
      brush.GradientStops[0].Color = Color.FromRgb(0,0,0);
    }
  }
}