A referee asks "are sunglasses permitted?"
Although it's no longer an official document of USSF, the answer can be found in Advice to Referees (2014)
5.1 Officiating Team Uniforms
All members of the referee team are expected to be in shirts which match as to color, design, and sleeve length. Socks are expected to match as to design and length. Shorts must be black and match as to leg length. Black shoes are preferred but referees can wear any color shoes so long as they are appropriate to perform the duties required of an official.
With the exception of watches, officiating team members should not wear any item of jewelry which would not be permitted to a player.
Hats or caps are not part of the officially accepted uniform and would not be worn at top competitive matches. Neither would glasses, tinted glasses, or sunglasses. Nothing should interfere with making eye contact between officials or with players.
Although almost 20 years old, this was also addressed in the USSF publication, October 1999 issue of Fair Play:
Q. May referees wear caps and sunglasses?
A. With regard to caps, the policy of the United States Soccer Federation was stated in the Spring 1994 issue of Fair Play magazine:
“Under normal circumstances, it is not acceptable for a game official to wear headgear, and it would never be seen on a high level regional, national or international competition. However, there may be rare circumstances in local competitions where head protection or sun visors might sensibly be tolerated for the good of the game, e.g. early morning or late afternoon games with sun in the officials’ line of sight causing vision difficulties; understaffed situations where an official with sensitive skin might be pressed into service for multiple games under strong sunlight or a referee who wears glasses needing shielding from rain.”
Sunglasses would be subject to the same considerations. In addition, we ask referees to remember that sunglasses have the unfortunate side effect of suggesting that the referee or assistant referee is severely visually impaired and should not be working the game. They also limit communication between the officials and the players by providing a barrier against eye-to-eye contact. Sunglasses, if worn, should be removed prior to any verbal communication with players.
Updated: April 2, 2017