I made a Wplace palette so you don’t have to. You can use it to transform a picture in the color used by Wplace. Just import the palette, use Filter - Map - Palettize, and then chose this palette.
Palette link: https://drive.google.com/file/d/1IhaMjohBze3NkXKIEJM-2qPt2SCU8ui3/view?usp=sharing