Проблема здесь в том, что Window.Background - это Brush, а SelectedColor и CurrentColor - это Color. Вы можете заставить это работать, используя Конвертер.
<Window x:Class="WpfApplication1.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:extToolkit="http://schemas.microsoft.com/winfx/2006/xaml/presentation/toolkit/extended"
xmlns:local="clr-namespace:WpfApplication1"
Title="MainWindow" Height="100" Width="200"
Name="Window" Background="blue">
<Window.Resources>
<local:BrushColorConverter x:Key="BrushColorConverter"/>
</Window.Resources>
<Grid>
<extToolkit:ColorPicker Name="colorPicker1"
SelectedColor="{Binding ElementName=Window,
Path=Background,
Converter={StaticResource BrushColorConverter}}"
CurrentColor="{Binding ElementName=Window,
Path=Background,
Converter={StaticResource BrushColorConverter}}" />
</Grid>
</Window>
и конвертер
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Windows.Data;
using System.Windows.Media;
namespace WpfApplication1
{
public class BrushColorConverter : IValueConverter
{
public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
SolidColorBrush brush = value as SolidColorBrush;
return brush.Color;
}
public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
Color color = (Color)value;
return new SolidColorBrush(color);
}
}
}