LuLu.com, Amazon KDP, and other on-demand book publishing platforms sometimes prefer RGB rather than CMYK color space PDFs. Apparently the thinking is it is best to allow the outsourced printing house to apply whichever CMYK conversion is most appropriate to the particular press that happens to run the job in question. The LuLu guidelines put a lot of emphasis on ensuring the press-ready PDF is "flattened" with no transparencies. I experimented with a large number of Affinity Publisher PDF