RGB.

The colour mode that describes each pixel as red, green and blue light, the way a screen makes colour, where CMYK describes ink. Most print-on-demand platforms want it: Printify does all its operations in RGB and warns that a CMYK upload is converted and the colours might shift significantly, and Printful says its printing systems are optimized for RGB files with an embedded sRGB profile. RGB names the mode, not the colour space. sRGB and Adobe RGB are both RGB, the same pixel values mean different colours in each, and the profile embedded in the file is the only record of which one was meant. That is why Printful asks sellers to confirm the profile is embedded before they upload. A file that is RGB but carries no profile leaves the receiving software to assume one, and the seller finds out which from the sample.