The color people will tell you that cyan and magenta do not equal red and blue. My university advisor tricked me into taking a 400 level class from the college of art and design on color theory. Really interesting class but an insane amount of work. Very early on the professor told us to throw out any book that identified red, yellow, and blue as the primary colors. It’s red, green, blue for light or cyan, magenta, yellow for pigment.
Yes, additive colour theory is based on red, green and blue (RGB). These are the colours you see if you look at your TV screen very closely.
Subtractive colour theory uses cyan, magenta and yellow. In printing black, abbreviated ‘K’, is added for contrast—CMYK. These are the inks used to print the dots you see if you look closely at a magazine photo.
I think people are confused by this because they’re taught a bastardised version of subtractive colour theory, using red, blue and yellow, at a very early age.
I'm saying that, with respect to color reproduction, paints work exactly the same as dyes and pigments. You can't make magenta paint from red, blue, and yellow. So the "primary colors" of paint are actually CMY.
Black in CMYK is not strictly necessary, you can absolutely make black out of CMY, but the separate ink gets added since black is such a regular occurence it's simply cheaper to not mix it out of the other colors.