The practice of wearing different colored socks is not permitted. As stated below in this USSF answer, the color of the sock is helpful to the referee and should all be the same color.
I have noticed lately a fashion trend in Girls Soccer using two different colored socks by the team ( i.e. orange and black; or white and orange etc.) I have researched all kind of information’s available to referees, but no answer found on rules identifying the used of matching sock only. High School Association identifies the situation as illegal equipment. NCAA only refers to matching uniforms and in contrast to the other team. FIFA only identifies socks.
For the referee sometimes the color of the sock is helpful in identifying a player submitting a rule violation in tackles or the like. Your advice is appreciated.
USSF answer (February 27, 2006):
There is indeed a requirement for uniformity of socks. While nothing is specifically written in Law 4 regarding the color of socks, tradition and common practice dictate that all members of a team (with the possible exception of the goalkeeper) wear socks of the same color, rather than each wearing his or her own choice or wearing socks of one color on one foot and socks of a different color on the other foot.
The ruling will be found in the USSF publication Advice to Referees on the Laws of the Game, which is based on the Laws, memoranda from FIFA and the International F. A. Board, and in memoranda and policy papers published by the United States Soccer Federation.
4.1 WEARING UNIFORMS
It is implicit in the Law that each side wear a distinctively colored jersey, that shorts and socks be uniform for each team, and that the uniforms be distinguishable from the uniforms worn by the other team. However, the details of the uniform are governed by the competition authority and can vary widely from one match to another. The referee must know and enforce the rules of each competition worked. Players’ jerseys must remain tucked inside their shorts, socks must remain pulled up, and each player must wear shinguards under the socks. Slide pants or similar undergarments must be as close as possible to the main color of the shorts.
Source: USSF’s Ask a Referee