The term "palette swap" comes from video games. A character would have a model called a sprite, which would have a certain coloring to it. In order to cheaply create new characters (especially enemy characters), a company would simply swap the color palette for a different one. Suddenly, it is a distinguishably different character, even though it looks and acts exactly the same as the old one. Think of old video games where there are ninjas in red costumes on level 1, and then on level 2, you fight ninjas in blue costumes.
Although it is a video game term, and it refers to a visual aspect, it is a great way to talk about cheap pseudocreativity in any field and form. And one of the universal warnings is that palette swaps aren't creative. They had a purpose in old video games and that was to save on the limited amount of space that was available. Outside of that, it s just being lazy.
As an example, think about comedy. You know what gets people to laugh? Seeing a guy get kicked in the balls. It may be crude and low-brow and all the other negative terms you have, but it gets people to laugh. However, you can't rely on it indefinitely. If every joke you have is kicking a guy in the balls, your audience will get bored of you rather quickly.
So now you need to spice things up. But how can you do that? Well, what if you tried punching a guy in the balls? It's all the comedy gold of ball-kicking, but now it involves a fresh fist instead of a stale foot.
I would not recommend that as a way to keep your audience interested. That is a prime example of a palette swap. The heart of the joke is still the same. The only thing different is the means to make it. You haven't made any changes. You haven't demanded the characters to act any differently, nor have you challenged the audience.
Although that is a simple analogy, it works for finer details too. Amateur writers always have characters smile or laugh during conversations. I understand why they do it: there is a certain impulse to have characters do more than just talk, and it's the initial reaction to most lines of dialogue. However, one does not improve sections of dialogue by replacing smiles and laughs with other physical tics. Rubbing hands or turning around are the exact same worthless actions that mean nothing in prose writing (and are only semi-decent directions for a movie).
Palette swaps aren't creative. They're lazy. And if you aren't sure if something you're doing is a palette swap, the quick rule is: Does replacing one thing for another affect the way people think/talk/react, and does it affect the story in any significant way?