Bambu has a guide on their wiki that i followed to create these files and print them. https://wiki.bambulab.com/en/knowledge-sharing/CMYK-color-lithophane-printing-instructions It has been a little while since i printed these but if i remember correctly each print is made up of 5 separate files that all need to get imported into the slicer together. Then you need to click yes to the check box asking whether to load these objects as a single object with multiple parts. After that you just need to assign the colours respective to what the file names are.

thanks a lot!