visible light - Is it only red, green and blue that can make up any color through additive mixture?

I'm reading about color vision and have some trouble understanding the motivation for why the trichromatic theory was suggested in the first place. The book I'm reading ("Psychgology: The science of mind and behavior") states:

At the beginning of the nineteenth century it was discovered that any colour in the visible spectrum could be produced by some combination of the wavelenghts that corresponds to the colours blue, green and red in what is known as additive color mixture.

From the explanation in the book, it seems like this somehow should be a reason for also supposing that the human retina was composed of cones sensitive to the colors green, red and blue respectively. I guess this would be a valid argument if it was only red, green and blue that, through additive mixture, could make up any color.

Is it only red, green and blue that, through additive mixture, can make up any color?

Answer

The human vision has 3 types of cones. (that is why all perception-based color spaces are 3 dimensional: LAB, XYZ, HSV ). Each cone type has a different sensitivity curve in the color spectrum (think of them as color filters). It gets complicated because these curves overlap: there isn't a single wavelength of light that triggers just one cone type.

So, in additive color synthesis, it would be nice if there would exist 3 colors that trigger the cones independently. By mixing light of these 3 base colors, you could create any color perception. But such a color set does not exist. RGB does a pretty good job of covering a large part of the color gamut, but not all (RGB fails at saturated cyan and yellow, for example).

In the XYZ space, you can see this quite visually: the total gamut has the shape of a horse footprint. An RGB color device can produce a triangle within. See the illustration in this question: https://stackoverflow.com/questions/2455503/cie-xyz-colorspace-do-i-have-rgba-or-xyza

The curved edge of this gamut representation are the monochromatic colors. These are the toughest to reproduce in additive color synthesis: you can only produce a monochromatic color by producing its exact peak spectrum.

You could consider adding a 4th and 5th primary color to an additive color reproduction system (e.g, an RGBCY monitor). That would be able to reproduce more colors, but apparently that has not been economical to this date.