There are currently four colors allowed by the USSF: Gold, Red, Black, and Blue.
More than a decade ago the referees always wore black and the teams were forbidden to wear that color. Since there was a strong desire by some teams to wear black and apparel manufacturers saw another way to make money, that was changed.
The teams were permitted to wear black and the referees switched to gold as a primary color. Today the latest instruction is that there is no primary color (yet the admin book still says gold is primary); all four are considered equal, and the referee team should select the one that best contrasts with what the teams are wearing. Ideally there should five different colors on the field: the two teams, the two GKs, and the refs. (At the highest levels the teams are required to change if there is a conflict with the referees, but at the lower and most youth levels the referees make the switch.)
PS The shirts come in both long and short sleeve, so I carry EIGHT shirts to matches!
PPS NCAA shirts are a totally different design and working that requires a referee to purchase eight more shirts!
