<b>VGS</b>

int c = 0;
             richTextBox1.SelectionStart = 0;
             richTextBox1.SelectionLength = richTextBox1.Text.Length;
             richTextBox1.SelectionColor = Color.Black;
             while (true)
             {
                 if (richTextBox1.Text.Contains(string textToFind))
                 {
                     if (c + 1 < richTextBox1.TextLength)
                     {
                         c = richTextBox1.Find(textBox1.Text, c + 1, RichTextBoxFinds.None);
                         if (c == -1) break;
                         richTextBox1.SelectionStart = c;
                         richTextBox1.SelectionLength = textBox1.Text.Length;
                         richTextBox1.SelectionColor = Color.Red;
                     }

                 }
                 else
                 {
                     MessageBox.Show("Не найдено");
                     break;
                 }
             }