BoolToColorConverter.cs

using System;
using System.Collections.Generic;
using System.Globalization;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Data;
using System.Windows.Media;

namespace Katas
{
    class BoolToColorConverter : IValueConverter
    {
        public static readonly BoolToColorConverter Default = new BoolToColorConverter();

        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
        {
            return (bool) value ? Brushes.Blue : Brushes.HotPink;
        }

        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
        {
            throw new NotSupportedException();
        }
    }
}

wpf7a.xaml.cs

<Window x:Class="Katas.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:Katas"
        mc:Ignorable="d"
        Title="MainWindow" Height="350" Width="525" DataContext="{Binding Source={RelativeSource Self}}">
    <Grid Background="{Binding IsChecked, ElementName=CheckBox, Converter={x:Static local:BoolToColorConverter.Default}}">
        <CheckBox x:Name="CheckBox" VerticalAlignment="Center" HorizontalAlignment="Center"/>
    </Grid>
</Window>