While opinions will differ on where Ben Affleck ranks on the list of cinematic Batmans, it's safe to say that his iteration of the Caped Crusader had one of the most faithful costumes last year in Batman v Superman: Dawn of Justice. For the character's big return in Justice League, costume designer Michael Wilkinson decided to go even further and make his suit a lighter gray compared to the Batman v Superman suit so that he looked like had been ripped straight from a comic book page even more so. Wilkinson explained his rationale with the following:
We wanted to honor the idea of a gray Batsuit as opposed to a black Batsuit. Batman in the graphic novels is usually drawn with a gray suit with a black cape as opposed to all in black. Also, Zack loved the idea that Batman's forearms would have this extra metallic plating strapped to it to collect hits and bullets and any kind of incoming.
Ever since Michael Keaton debuted as Batman in the 1989 Tim Burton movie, the DC Comics hero has typically fought crime in an all-black Batsuit in a theatrical setting, though we'd be remiss if we didn't mention the Joel Schumacher period where both Val Kilmer and George Clooney's iterations would blue-colored Batsuits. In any case, the DCEU's Batman is the closest the character has come to looking like his comic book predecessor, who usually wears a gray/blue or gray/black combo (yellow oval for the Bat-symbol optional). In addition to the faithful color coordination, it was also a good call to include the extra plating since it never hurts to have extra protection in Batman's line of work.
Warning: SPOILERS for Justice League are ahead!
Of course, this is Batman we're talking about, so the man has other Batsuits tucked away for different kinds of missions. For Justice League, Bruce Wayne changed out of his normal duds after getting a beatdown from Superman and switched to a black-colored tactical suit so he'd be properly protected during the final battle against Steppenwolf and his Parademon army. Here's what Michael Wilkinson had to say about this suit while speaking with Fashionista:
Batman faces a challenge of even higher intensity in this film, so we wanted to give him what we call a 'tactical suit' for high level battle for when things are becoming more serious and deadly. This suit is has exterior layers of armor and extra details and it has a different cowl. He has additional eye protection.
Batman held his own against Parademons earlier in Justice League, but it was a wise decision to change into the tactical suit when he and his allies went to stop Steppenwolf for good. Now that Batman dedicated to keeping the Justice League together, no doubt he'll start working on other special Batsuits for the day that another invading alien force or a different kind of extraordinary menace threatens humanity.