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. More info, from Barça's website:
In an effort to improve visibility and clearly differentiate the teams for both referees and spectators, UEFA requested that FC Barcelona and Atlético Madrid wear their second kits for both games of their upcoming Champions League quarter-final tie. Barça, thus, will be wearing their red and gold 'Senyera' kit, while Atlético will be sporting dark blue.
A similar situation occurred earlier this season when FC Barcelona faced Twente in the second leg of the UEFA Women's Champions League. Since the Dutch side could only choose from all red or all blue, Barça played with the 'Senyera' kit at their home grounds.
This will not be the first time Barça have to play with their away kit at Camp Nou in the Champions League. Back in the 2008/09 season, the Catalans played Wisla Krakow with a yellow/gold kit as well. They won 4-0 and later became champions in Rome. This obviously means nothing to the game against Atleti, but it's not bad to know that we have won with an away kit at home just before doing it again.
It would certainly be better to see Barça playing in Blaugrana at their home ground, but nobody is against clearly seeing which team is which.