<?xml version="1.0"?>
<Canvas
   xmlns="http://schemas.microsoft.com/client/2007"
   xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">

  <Rectangle Width="400" Height="400" >
    <Rectangle.Fill>
      <RadialGradientBrush GradientOrigin="1,0">
        <GradientStop Color="Yellow" Offset="0.0" />
        <GradientStop Color="Red" Offset="0.25" />
        <GradientStop Color="Blue" Offset="0.75" />
        <GradientStop Color="LimeGreen" Offset="1.0" />
      </RadialGradientBrush>
    </Rectangle.Fill>
  </Rectangle>
</Canvas>