I always thought they wore red while home and white while away. However, when I saw the highlights of their first game (2014 season) against the Cowboys on 8 September 2014, I noticed them wearing red even though they were playing at the AT & T Stadium in Dallas! Shouldn't it have been white since they weren't playing in Santa Clara?
For each game, the home team decides whether they want to wear colors or white, and the visiting team wears the opposite. The majority of teams wear colors at home, which means that the visiting teams wear white. However, Dallas traditionally wears white at home, which means that usually teams that go to Dallas to play wear their color uniforms. As a result, Dallas wears white uniforms in almost every game they play, both home and away.
Dallas isn't the only team to select white jerseys at home. Washington wore white at home for many years, and Miami usually chooses white for day games, forcing their opponents to wear dark colors in the Florida sun.
I think I just got an answer to this question elsewhere but I'll go ahead and post it for everyone's benefit anyways:
Dallas has traditionally dictated the Cowboys wear white at home. Consequently, the visiting team wears its home colors while playing the Cowboys in their home stadium.