Can a Coach use Signs on sideline?

[Schmidt MJ]

Last year we had a team do this type of play calling. The offense never huddled up. They always lined up at the LOS and all players looked over to the sideline and one coach held up a "color" and another coach held up a "number". The players all had the plays taped to their forearms. If the play was "blue 66" then all the players looked at their play list and new what the play was.