It's been popular lately to assign a color theme to a wedding and ask your guests to dress in the color of your choice. "White weddings," where everybody (the wedding party and guests) wears white, are also trendy, especially in tropical destinations. In my experience as a wedding planner, my clients who have had color-themed weddings all said they did it for the monochrome pictures, which they've seen featured everywhere and loved.
However, there are some realities to a "white wedding" that should not be overlooked by the couple when they do their planning.
First and foremost, you need to make sure that all of the guests understand what the attire is. It's never safe to assume anything for a wedding—especially when you're asking literally everyone to break with etiquette and wear the bride's color of the day to her wedding. While traditionally, putting "formal" or "semi-formal" on the invitation is sufficient guidance, you have to take it a step further when you're telling your guests what color you want them to wear.
If your invitation is non-traditional and includes a lot of information about your wedding, you should definitely include it there. You can phrase it one of the following ways:
The bride and groom request that all guests wear white attire to their wedding ceremony and reception.
It's a white wedding—please embrace the theme and wear something white!
Reality Check: No matter how well you explain your "white wedding" theme to your family and guests, not everybody is going to wear all white. In fact, the vast majority of your gentlemen guests will wear khaki pants and a white shirt. And most likely no man outside the wedding party will have white shoes unless you encourage them to wear sneakers. "White only" becomes white-from-the-waist-up to most of the men. Those who are willing to wear white pants probably already own some, and will rock them with their white shoes.
If you're having an outside, sunset wedding, beware of see-through fabrics! You won't want to dress the men in your wedding party in linen pants that become see-through. Also, make sure the dresses for the ladies are lined. While a cool linen shift may be exactly the look you're going for, be certain the dress is ALL the guests will be seeing during the ceremony. Make sure to remind your entire wedding party to wear only white or nude undergarments so nothing shows under the lights during pictures.
Lastly, don't get upset if a few of your guests forget the dress code. Some guests will likely wear white with a pattern, or no white at all—if they've got a clue, they'll make a point to step out of some of your formal group photos so the color theme isn't ruined. The truth is, when a wedding isn't black tie, most people don't see the need to purchase a new outfit they'll literally never wear again for one day. Expect (and hope for) bright whites and you'll be pleasantly surprised if everybody remembers.