The home team gets to pick their color of jersey and the visiting team has no choice but to wear the contrasting color.
In the late 60's and early '70's the Rams wore white so the visiting team had to wear dark, not because the Rams wanted the lighter shade in the sunny afternoons, but because the owner wanted LA fans to be able to see the other teams colors throughout the season. In 1973 on, the Rams wore dark blue and visitors the whites with one huge exception starting the late '70's--the Rams wore whites when the Cowboys were in town because of the supposed dark jersey jinx associated with them.
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