The first and most important rule is no white. I'd take it as far as nixing cream and very light nude tones, as well. Aside from that, and not wearing anything too revealing, modern wedding guest attire is pretty flexible. The main factor is knowing whether the wedding will be a vintagey daytime soiree or a black-tie evening gala - or something in between.

When in doubt, choose a colorful print. Florals are always nice for summer weddings, but prints in general almost always ensure that you won't be mistaken for a bridesmaid. In this piece, we chose all fitted silhouettes, which are sophisticated, and they instantly make every dress a little dressier. Remember, for such a special occasion, slightly overdressed is better than being underdressed. After all, your outfit will live in Facebook photos for years to come. 

1. Topshop Rainbow Floral Peplum Dress, $96

2. Wren Sequin Shift Dress, $495 

3. Vivienne Westwood Gold Label Long Ball Printed Silk Gown, $3,805

4. Yoana Baraschi Sunblaze Lace Dress, $328

5. McQ by Alexander McQueen Lace-Print Stretch-Jersey Dress, $405

6. J. Crew Ella Dress, $199.99

7. Alice + Olivia Venus Strapless Dress, $440

Lily Ko (lily.7x7 @ gmail.com) studied fashion design and merchandising at Central Saint Martins and the Academy of Art University. In addition to writing for 7x7, Lily is on the creative team at Pottery Barn, and also works as a freelance stylist and apparel designer.