Use Named color to paint Rectangle
<Window x:Class="WpfApplication1.Window1" xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" Title="WPF" Height="300" Width="300"> <Canvas Margin="5"> <!--Named color--> <Rectangle Canvas.Top="10" Canvas.Left="50" Fill="RoyalBlue" Height="70" Width="230" /> </Canvas> </Window>