It looks like Jason Mamoa’s Aquaman could have had a very different look in the Justice League movie. Concept art from Constantine Sekeris reveals a gold and green suite more similar to what readers are used to seeing in the comics.
While it’s not known why this version of the costume was scrapped, there are some theories. After the disappointing ticket sales of Batman v. Superman: Dawn of Justice, which failed to hit $1 billion worldwide despite the fact that it should easily have done so given that it starred two major superheroes, it’s possible that Snyder and the Justice League team were trying to help the DC Extended Universe course correct.
Batman v. Superman was widely criticized for being too dark and joyless. A more colorful Aquaman costume likely would not fit for a Justice League film aiming for a similar tone.
At the same time, fans seemed to be having trouble envisioning a “cool” Aquaman, as the character has sometimes been considered somewhat geeky in comparison to the DC Universe’s other superheroes. A more toned down costume would likely help with some of the character's potential image issues. In addition, Snyder’s departure from Justice League and his being replaced by Joss Whedon may have had something to do with the costume change as well.
It is also worth mentioning that the costume seen in Sekeris’s concept art is similar to the one Momoa wears in Aquaman’s final act. So it’s possible this version, which is more faithful to what fans have seen in the comic books, was being reserved for the character’s solo movie.