In addition to this, there will always be 2 sets of "primary" colors for a given eye: Additive and Subtractive.

Additive primary colors are necessary when you have no light, and need to create color. Think a black screen, and you are creating colors with RGB pixels.

Subtractive primary colors are necessary when you have full-spectrum (white) light and need to filter down to a single color.

Other "primary" colors, such as the red, blue, yellow pigment primaries we learned in Kindergarten exist because pigments historical couldn't be created perfectly, and those "primaries" are the best way of getting the most colors, but still have a very limited (by comparison) gamut.

Yes, one challenge with defining Subtractive primaries is that they are dependent on the white point of the "white" light source (e.g. D65 vs D50). While this seems inconvenient, it's worth noting that the apparent color of greys for Additive primaries is also dependent on surrounding illumination.

So primaries are useful for generating roughly orthogonal changes in perceived color, but they don't tell you how they will be perceived in absolute terms without knowing surrounding illumination. In the simplest case, asking if something is bright (even without color) is impossible without knowing the surroundings.

> There will always be 2 sets of "primary" colors for a given eye: Additive and Subtractive.

If your eye only has two types of cone cells then your additive and subtractive primaries are the same ;)