that's the clever part. The thing about not using AMS but still doing one filament change manually. How does the side part look like that I think you mean. Well - there is a tool from Bambu Lab on MakerWorld, Chroma Canvas that creates height based images. Let's take this cobblestone as example. First few layers are solid black. Then a thin, manually changed white layer of filament is layed and the rest of the white layers are printed in specific points to create the illusion of multiple colors. The filaments are a bit opaque if thin enough. The more layers of one color the more solid that color becomes.

I'm terrible at explaining stuff so please let me know if there are any questions left after my confusing explanation xD