This reminds me of Impossible colors (https://en.wikipedia.org/wiki/Impossible_color). In this example, the set is imaginary because it requires negative values of some primary colors. However, for impossible colors, the set is not possible because there is overlap between the wavelengths that each cone can detect; if you could theoretically only stimulate the green cone in isolation, then you could get a greener green than what is physically possible.
This reminds me of Impossible colors (https://en.wikipedia.org/wiki/Impossible_color). In this example, the set is imaginary because it requires negative values of some primary colors. However, for impossible colors, the set is not possible because there is overlap between the wavelengths that each cone can detect; if you could theoretically only stimulate the green cone in isolation, then you could get a greener green than what is physically possible.