Set your content to be HTML:
MailMessage mail = new MailMessage();
mail.Body = body;
mail.IsBodyHtml = true;
Then just specify the colour:
mailResult.AppendLine("<font color=\"red\">Previous Day Management Data Count: " + MgmtTotalDataCount + "</font>");
The result is:
Previous Day Management Data Count: 666
[edit]Forgot to escape the double quotes :doh: - OriginalGriff[/edit]