Most films are linear, meaning time moves forward throughout the film. For example, most films tend to start with the character or story's journey and then move forward through time as certain events play out.
This is not the case with "Memento"; it is one of Christopher Nolan's earlier films.
It was the first of his I saw, and much like his other famous features, "Oppenheimer," "Inception," and "Tenet," there is often some sort of playing with time, multiple timelines of the same events or specific flashbacks from different perspectives.
There is a big upside and a big downside to going down these routes of using time as a tool in your filmmaking. The plus is that it creates a unique perspective and a very different approach to telling and showing a story, which can be very engaging. The downside is that if not done correctly, it can be incredibly confusing for the viewer or audience, thus making the story difficult to follow and not moving your intended audience.
In "Memento", there is a direct visual disparity that helps the audience understand the timeline of the film. The black and white film mixed with colour makes up the difference in this disparity; when we see visuals in black and white, the events are unfolding in a forward order (how they are in the world). When the scenes are in colour, it is the reverse; you're seeing the visuals from the end to the beginning.
The plot of the film is around the main character, Leonard, who has anterograde amnesia, which means he cannot create new memories. He can only remember that someone killed his wife and that he must find and get revenge. He uses notes, polaroids and tattoos to remind himself of details to eventually find the person who killed his wife.
The confusing way of a timeline, using reverse film, flashbacks, and different coloured footage, makes the audience feel similar to the character each day, constantly confused about what's real or true because of his memory problem.
As we move through the film, the thrilling mystery of who the killer is and tracking them down is just as confusing for the main character as it is for us, the audience. I think the technique of utilizing black and white with colour is very interesting and is a much more immersive way to show different timelines or scenes at different times of the plots, unlike a standardized "flashback."
Key takeaways from this film:
Wait to see the big picture. Due to Leonard's memory problem, he often chased down incorrect paths or jumped to conclusions about people's intentions. And sometimes, when you think you've got the right thing, you can be the furthest from it.
Trust; be careful who you trust. Leonard often found himself vulnerable or taken advantage of due to his memory problem.
Reverse engineer a problem. This is ironic because a lot of the film is in reverse order, sometimes when trying to find a solution to something you have to work your way back from the source.