UPDATE: Sister missionaries in approve areas are now allowed to wear pants to prevent disease.
In the past, the rules regarding sunglasses for missionaries were a bit unclear. Generally, it was accepted that sunglasses were only worn while driving in a car or on preparation day. Now, in order to ensure the protection and health of missionaries (especially in hot climates), the Church has updated the clothing guidelines to make it clear missionaries are allowed to wear sunglasses during any phase of their proselyting work so long as they meet the standards.
The following examples were given on LDS.org of appropriate sunglasses for both elders and sisters.
Elders
Sisters
You can see all the dress and grooming guidelines here.

Aleah is a graduate of Southern Virginia University, where she studied English, Creative Writing, and Dance. She now works full time as a marketing and product manager, writer, and editor. Aleah served a mission in California and loves baking, Lang Leav poetry, Gaynor Minden pointe shoes, and Bollywood movies.