The club tried to play with the usual white uniform but Manchester United refused the idea of playing with their black shorts again. The UEFA then decided to make Real Madrid play with the away uniform in the Champions League.
So far, Real Madrid haven't been lucky with the green kit, and that's why the club wanted to play with the white one. If that was to happen, Manchester United had to accept changing their white shorts to black, but the British club declined that option.
Real Madrid knocked out Manchester United the last two times playing with the away kit. Black in 2000 and blue navy in 2003.