// install Extended.Wpf.Toolkit

xmlns:xctk="http://schemas.xceed.com/wpf/xaml/toolkit"



private void ClrPcker_Background_SelectedColorChanged(object sender, RoutedPropertyChangedEventArgs e)
{
    TextBox.Text = "#" + ClrPcker_Background.SelectedColor.R.ToString() + ClrPcker_Background.SelectedColor.G.ToString() + ClrPcker_Background.SelectedColor.B.ToString();
}