The Church recently updated their dress and grooming guidelines for missionaries, allowing missionaries to wear conservative sunglasses and wide-brimmed hats in order to help them better protect against the sun.
The Church holds its missionaries to high standards—both spiritually and when it comes to dress and appearance. As servants of the Lord, they represent Him in every way.
Missionaries are told, “as an ambassador of the Lord you are to wear professional, conservative clothing that is consistent with your sacred calling and that will clearly identify you as a missionary for The Church of Jesus Christ of Latter-day Saints” (Dress and Grooming Guidelines for Missionaries).
Read about even more updates that have happened over the years at LDS Living.