Which principle explains why the identification of primary colours can change when the medium changes?

Correct answer: The colour-producing process depends on the medium

Primary colours are defined by the colour-producing system, not by one universal list. In additive light mixing, screens use red, green and blue because coloured light is combined by emission. In traditional subtractive pigment mixing, teaching systems commonly use red, yellow and blue, or more precise printing systems use cyan, magenta and yellow. Thus the medium and its physical process determine the relevant primaries.