problem with project

A WPF rectangle can only have one colour which is defined by the Fill property.
You could write a User Control which consists of three rectangles. e.g.

SpecialRectangle.xaml:

<UserControl x:Class="WpfApplication9.SpecialRectangle"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             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/blend/2008" 
             mc:Ignorable="d" 
             d:DesignHeight="300" d:DesignWidth="300">
    <Grid>
        <Grid.ColumnDefinitions>
            <ColumnDefinition></ColumnDefinition>
            <ColumnDefinition></ColumnDefinition>
            <ColumnDefinition></ColumnDefinition>
        </Grid.ColumnDefinitions>
        <Rectangle Grid.Column="0" Fill="Blue"></Rectangle>
        <Rectangle Grid.Column="1" Fill="Red"></Rectangle>
        <Rectangle Grid.Column="2" Fill="Green"></Rectangle>
    </Grid>
</UserControl>

You could then use this on any window like this:

MainWindow.xaml:

<Window x:Class="WpfApplication9.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:local="clr-namespace:WpfApplication9"
        Title="MainWindow" Height="400" Width="525">
    <Grid>            
        <local:SpecialRectangle></local:SpecialRectangle>
    </Grid>
</Window>