I never thought about whether or not an artist’s palette was preserved. I’m glad someone did. The Guardian has a great article titled “Why preserve Van Gogh’s palette?”
Seurat stuck rigidly to colour theory for his painting La Grand Jatte. You can see from his palette, above, that he obeyed the theory of Cheuvreul. He believed the basic colours red, yellow and blue reach the eye with different wavelengths and are mixed on the retina of the eye. Consequently Seurat kept his pigments in order on the palette, only adding the complimentary colour (red/green or violet/yellow), which is what creates the impression of fizzing light, on the canvas itself. Black was outlawed, since it was defined by the physicists as non-light.