Simple question: what is the "ground truth"? If you author an image on Display A and save its colours, then how does Display B know to render colours correctly

Simple question: what is the "ground truth"? If you author an image on Display A and save its colours, then how does Display B know to render colours correctly that looks consistently with Display A? You have two options: 1.) Tag Display A's colour space into the image, then convert colours to match Display B's colour space when rendering; 2.) Convert colour data from Display A colour space into a generic colour space (like sRGB, Display P3, ACEScg, etc) and embed that in to the image, then convert colours to match Display B's colour space when rendering.