Selectively coloring text in RichTextBox

Try this:

static void HighlightPhrase(RichTextBox box, string phrase, Color color) {
  int pos = box.SelectionStart;
  string s = box.Text;
  for (int ix = 0; ; ) {
    int jx = s.IndexOf(phrase, ix, StringComparison.CurrentCultureIgnoreCase);
    if (jx < 0) break;
    box.SelectionStart = jx;
    box.SelectionLength = phrase.Length;
    box.SelectionColor = color;
    ix = jx + 1;
  }
  box.SelectionStart = pos;
  box.SelectionLength = 0;
}

…

private void button1_Click(object sender, EventArgs e) {
  richTextBox1.Text = "Aardvarks are strange animals";
  HighlightPhrase(richTextBox1, "a", Color.Red);
}