Imagine you had a white canvas and only four paints to work with: seafoam, lilac, khaki, and black. You can mix them however you want, and apply as thinly or thickly as you want, and you can get colors from all over the rainbow-- but no matter what you do, they’re going to be desaturated, washed-out, gray versions of the colors. You’re never going to get really saturated colors out of those paints-- no neon red, lime green, or electric blue.
All pigments (and combinations of phosphors) are like that-- it’s just that some are more so than others. We can compare all the colors reproducible with a given scheme to the colors the eye is capable of perceiving by examining the color space the combination of pigments is capable of reaching. CMYK is the little irregular pentagon in the middle; it can’t access much color space at all.
