No clue why I want to be vague about this. Probably because I'm late for a party, but really wanted to share.

And you're smart. You'll figure it out.

<Window x:Class="MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="VS Copy-Paste to HTML" >
    <Grid>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="*" />
            <ColumnDefinition Width="*" />
        </Grid.ColumnDefinitions>
        <RichTextBox Name="m_richTextBox" TextChanged="m_richTextBox_TextChanged" />
        <TextBox Grid.Column="1" Name="m_textBoxHTML" />
    </Grid>
</Window>

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Web;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Documents;
using System.Windows.Media;

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
    }

    private void m_richTextBox_TextChanged(object sender, TextChangedEventArgs e)
    {
        m_textBoxHTML.Clear();
        m_textBoxHTML.AppendText("<div style=\"font-family:monospace;\">" + Environment.NewLine);
        foreach (Paragraph paragraph in m_richTextBox.Document.Blocks.Cast<Paragraph>())
        {
            foreach (Run run in Decompose(paragraph.Inlines))
            {
                m_textBoxHTML.AppendText(toCleanedColoredSpan(run));
            }
            m_textBoxHTML.AppendText("<br/>" + Environment.NewLine);
        }
        m_textBoxHTML.AppendText("</div>");
    }

    private string toCleanedColoredSpan(Run run)
    {
        SolidColorBrush solidColorBrush = run.Foreground as SolidColorBrush;
        if (solidColorBrush == null)
        {
            return cleanString(run.Text);
        }
        else
        {
            if (solidColorBrush.Color == Colors.Black)
            {
                return cleanString(run.Text);
            }
            else
            {
                string colorString = solidColorBrush.Color.ToString();
                Debug.Assert(colorString.Length == 9);

                colorString = colorString.Substring(3);

                return string.Format("<span style=\"color:#{0};\">{1}</span>",
                    colorString, cleanString(run.Text));
            }
        }
    }

    private string cleanString(string source)
    {
        source = HttpUtility.HtmlEncode(source);
        source = source.Replace(" ", "&nbsp;");
        return source;
    }

    private static IEnumerable<Run> Decompose(IEnumerable<Inline> inlines)
    {
        foreach (Inline inline in inlines)
        {
            if (inline is Span)
            {
                foreach (Run run in Decompose(((Span)inline).Inlines))
                {
                    yield return run;
                }
            }
            else if (inline is Run)
            {
                yield return (Run)inline;
            }
            else
            {
                throw new NotSupportedException();
            }
        }
    }
}