I\'ve got a WPF TextBox into which my users type XHTML.
XAML...
Like Kent said with a TextBox you're stuck with one color.
Here is a small article on simple RTB syntax highlighting that should give you most of what you need.
http://blogs.microsoft.co.il/blogs/tamir/archive/2006/12/14/RichTextBox-syntax-highlighting.aspx
You'll want to use a RichTextBox for that.