If you ever had trouble watching Barcelona and Atlético Madrid play because they uniforms look similar, that will not be a problem.
UEFA has requested both teams to wear their away kits in their Champions League quarterfinal matchup, at Camp Nou and the Vicente Calderón, specifically Barça's yellow (or gold) kit with a Senyera in the back, and Atleti's beautiful dark blue outfit.
The reason for the ruling is simple: it was hard to tell them apart on TV in La Liga games this season, and UEFA wants to solve that problem.