Oppenheimer is Christopher Nolan’s longest film to date and his R-rated film in over 20 years. Apart from that, what makes Oppenheimer unique is that it has both color and black-and-white scenes.
Oppenheimer is a biographical film about the story of J. Robert Oppenheimer and his role in the development of the atomic bomb. Throughout the movie, the scenes switch between color and black and white.
Talking about why Oppenheimer switches from color to black-and-white, director Nolan revealed that it’s because the film has two timelines, and the switch is a change in perspective.

In an interview with the Associated Press, Nolan said:
Additionally, speaking to Total Film (via GamesRadar), Nolan said:
So, the use of both color and black-and-white scenes in Oppenheimer represents a change in perspective. As the scenes switch from color to black-and-white, so do the perspectives.
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The Color Scenes in Oppenheimer Are Subjective

Nolan revealed that he wrote the script for Oppenheimer in first-person, which is the point of view for the color scenes. Nolan shared with Empire:
So, the color scenes in Oppenheimer are subjective and more personal. It’s the story told from Oppenheimer’s perspective, which is “the bulk of the film.”
Notably, the film does not just show Oppenheimer’s role in the development of the atomic bomb. It also shows what’s going on inside his head and the reasoning for his actions and decisions.
The film even shows aspects of the personal life of the “father of the atomic bomb.” All of these scenes are in color, which means that the color scenes are more personal, and indeed, the story is told from Oppenheimer’s point of view.
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The Black-and-White Scenes in Oppenheimer Are Objective

When Oppenheimer switches to black-and-white, it shifts into an objective view of the story. As Nolan mentioned, “It's a more objective view of his story from a different character's point of view.”
Notably, the scenes that are in black-and-white are those that involve Lewis Strauss (Robert Downey Jr.), Oppenheimer's rival.
The black-and-white scenes show the story from Lewis Strauss' perspective, not Oppenheimer's, therefore making it an objective view.
So, a switch from color to black-and-white shows a change in perspective, which makes Oppenheimer a unique film that’s worth watching.