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
For Each ctrl As Control In Me.Controls("pnlMainPanel").Controls
If ctrl.GetType Is GetType(System.Windows.Forms.Panel) Then
For Each subCtrl As Control In ctrl.Controls
If subCtrl.GetType Is GetType(System.Windows.Forms.TextBox) Then
If subCtrl.GetType Is ...