Why didn't you asked that question in the first place?
I'm affraid it can't be done in a normal textbox.
But it can certainly be done in a RichTextBox.
I have not tried it but this should work:
MyRichTextBox.Select(6,1);//select char 6 from my text
MyRichTextBox.SelectionColor=Color.Red;//color this red
Open your help menu and choose search, type RichTextBox for more information