CMYK.

The colour mode that describes a pixel as amounts of cyan, magenta, yellow and black ink. Every print path ends there, often with white or extra inks added, so the argument is only about who converts. Printful says converting before upload can unnecessarily reduce the available color information, and Printify tells sellers to preview in CMYK and then undo the change. Some DTF shops want the reverse: Transfer Kingdom requires CMYK and auto-converts RGB with a warning that colours may differ, and Transfer Superstars asks for a FOGRA39 conversion before submitting. A PNG cannot hold CMYK at all, as Fourthwall points out, so a shop asking for CMYK is also asking for a different file format. Whoever converts first decides the gamut, and a file converted for one shop and reused at another carries that loss to both.