Change the color of text in richtextbox uisng string builder

Please am looking for away to change the color of text retrieved from database(sql server) using string builder, I have tried several methods but did not work, can anyone help me out.

SqlCommand cmd= new SqlCommand("SELECT * FROM [Box] where item=@item", connect);
cmd.Parameters.AddWithValue("@item", item.Text);
StringBuilder resp = new StringBuilder();
SqlDataReader cmd_reader = cmd.ExecuteReader();
item.Text = "";
while (cmd_reader.Read())
{
respond.SelectionColor = Color.Red;
resp.Append(cmd_reader.GetString(6)).Append(Environment.NewLine);
respond.SelectionColor = Color.Green;
resp.Append(cmd_reader.GetString(7)).Append(Environment.NewLine);
}
item.Text = resp.ToString();

is retrieving fine from the database but I want to change the colors of each line..