Apparel

From DCFanguide

The team encourages all fans to show their support! Most DC United fans wear black, with maybe a little red and white, just like the players on the field. Official team apparel is available through the DC United website or on gamedays at merchandise stands located in the concourses at RFK, but just your basic black will do in a pinch. It goes with anything, it makes you look skinny, and all the cool kids are wearing it.

One note of caution: if black isn't your color, be careful about wearing the colors of the other team. We wouldn't want those guys looking into the stands and inadvertently being encouraged by seeing you clad in the wrong color.