Set the SelectionFont property to an appropriate font.

To enable users to set the font family, size, and typeface in an application, you would typically use the FontDialog component. For an overview, see FontDialog Component Overview (Windows Forms).

Set the SelectionColor property to an appropriate color.

To enable users to set the color in an application, you would typically use the ColorDialog component. For an overview, see ColorDialog Component Overview (Windows Forms).

RichTextBox1.SelectionFont = New Font("Tahoma", 12, FontStyle.Bold)
RichTextBox1.SelectionColor = System.Drawing.Color.Red

richTextBox1.SelectionFont = new Font("Tahoma", 12, FontStyle.Bold);
richTextBox1.SelectionColor = System.Drawing.Color.Red;

richTextBox1->SelectionFont =
   gcnew System::Drawing::Font("Tahoma", 12, FontStyle::Bold);
richTextBox1->SelectionColor = System::Drawing::Color::Red;

Note:
These properties only affect selected text, or, if no text is selected, the text that is typed at the current location of the insertion point. For information on selecting text programmatically, see TextBoxBase.Select Method.