If I understand correct, you seek to highlight some text while sending an email.
For highlighting text via some color, you would need to use HTML or RTF format of text. This would enable you to have text formatting that can be sent in an email.
Look at similar query answered before: How to send Color content as a email using smtp in C#?
Simplest though would be marking the mail message body as HTML and assigning the needed text to it in html format as per need.
msg = new MailMessage("firstname.lastname@example.org",
"email@example.com", "Message from CodeProject",
"This email sent by the CodeProject<br />" +
"this is bold text!");
msg.IsBodyHtml = true;
Now, for any reason you were looking for having multiple colors for a text of ONE label. That would not be possible. A label has single color.