public class CustomControl1 : Button
{
    public CustomControl1()
    {
        if (System.ComponentModel.DesignerProperties.GetIsInDesignMode(this))
        {
            Background = Brushes.Red;
        }
    }
}

namespace InlineEditorNamespace
{
    using System;
    using System.Collections.Generic;
    using System.Text;
    using System.Windows;
    public partial class EditorResources : ResourceDictionary {
        public EditorResources()
            : base()
        {
            InitializeComponent();
        }
    }
}

<ResourceDictionary
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:PropertyEditing="clr-namespace:Microsoft.Windows.Design.PropertyEditing;assembly=Microsoft.Windows.Design"
    xmlns:Media="clr-namespace:System.Windows.Media;assembly=PresentationCore"
    xmlns:sys="clr-namespace:System;assembly=mscorlib"
    x:Class="InlineEditorNamespace.EditorResources">
    <DataTemplate x:Key="BrushInlineEditorTemplate">
        <Grid>
            <Grid.ColumnDefinitions>
                <ColumnDefinition Width="1*"/>
                <ColumnDefinition Width="Auto"/>
            </Grid.ColumnDefinitions>
            <ComboBox Grid.Column="0" Text="{Binding StringValue}">
                <ComboBoxItem>Red</ComboBoxItem>
                <ComboBoxItem>Blue</ComboBoxItem>
                <ComboBoxItem>Green</ComboBoxItem>
            </ComboBox>
        </Grid>
    </DataTemplate>
</ResourceDictionary>

namespace InlineEditorNamespace
{
    using System;
    using System.Collections.Generic;
    using System.Text;
    using Microsoft.Windows.Design.PropertyEditing;
    using System.Windows;
    using InlineEditorNamespace;

    public class BrushInlineEditor : PropertyValueEditor
    {
        private EditorResources res = new EditorResources();

        public BrushInlineEditor()
        {
            this.InlineEditorTemplate = res["BrushInlineEditorTemplate"] as DataTemplate;
        }
    }
}

namespace InlineEditorNamespace
{
    using System;
    using System.Collections.Generic;
    using System.Text;
    using Microsoft.Windows.Design.Metadata;
    using System.ComponentModel;
    using Microsoft.Windows.Design.PropertyEditing;
    using System.Windows.Media;
    using System.Windows.Controls;
    using System.Windows;
    using CustomControlLibrary;

    // Container for any general design-time metadata that we want to initialize.
    // Designers will look for a type in the design-time assembly that implements IRegisterMetadata.
    // If found, they will instantiate it and call its Register() method automatically.
    internal class Metadata : IRegisterMetadata
    {

        // Called by Cider to register any design-time metadata
        public void Register()
        {
            AttributeTableBuilder builder = new AttributeTableBuilder();
            builder.AddCustomAttributes
                (typeof(CustomControl1), 
                Control.BackgroundProperty,
                PropertyValueEditor.CreateEditorAttribute(
                    typeof(BrushInlineEditor)));
            MetadataStore.AddAttributeTable(builder.CreateTable());
        }
    }
}

<Window x:Class="DemoApplication.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="Window1" Height="300" Width="300" xmlns:my="clr-namespace:CustomControlLibrary;assembly=CustomControlLibrary">
    <Grid>
        <my:CustomControl1 Margin="30,30,30,30" Name="customControl11">Button</my:CustomControl1>
    </Grid>
</Window>