When a couple coordinate their style and clothing, they can certainly make a statement. Sometimes men will wear a tuxedo or a suit, and the woman will wear a dress of her choice. But in situation where a couple want to complement each other with what they're wearing, a formal style of clothing makes it easy.
Black is often considered to be a formal color, as in a black tie event. Here, men will wear a tuxedo and women a long or formal dress. The couple can both wear black. If the man is wearing a white shirt with a black tuxedo, the woman could wear a black dress with a pop of white, such as a sash or scarf. This will look formal and coordinated with all black and a splash of white for both.
If it's an ultraformal event, also known as a white tie event, it will call for men to wear a white shirt, white bow tie, white waistcoat and a black tailcoat. Another idea for a couple is for the woman to wear all-white to complement the white accents of his suit. She would wear a white formal dress, complete with white shoes. Naturally, it would depend on the event as to whether it's appropriate for the woman to wear white. For example, even if it's a formal wedding, the bride should be the one wearing the white dress.
In the formal event that calls for the woman to wear a cocktail dress, it could be a stylish idea to match the dress with the man's tie. For example, if she wears a bright red cocktail dress, he could wear an all black suit with a bright red tie. This is a way of coordinating a stylish look even though they are not wearing the same color. The dash of color with the red tie will complement her dress and look pulled together as a couple.
If it's a business formal event, a one-color, well-tailored suit is the expected attire for men. In this case, dark brown or navy blue is acceptable in addition to black. As a couple, one idea would be for him to wear a navy blue suit with a light blue tie and she could wear a light blue dress. The different shades of blue being worn together would make a tasteful fashion statement.