Standard displays already vary the intensity of their red, green, and blue subpixels; the color itself is produced by mixing those primaries. The subpixels are fixed-wavelength emitters or filters, but their brightness is not simply on/off.

For human vision, RGB works because our color perception is largely based on three types of cone cells with overlapping sensitivity curves. That means color is effectively three-dimensional for normal viewing, so three suitably chosen primaries can reproduce a very large range of perceived colors.

A tunable “any-wavelength” pixel could in principle help extend gamut in some cases, because real RGB primaries do not cover every color we can perceive and every display has limits. But such a system would be far more complex, and for normal human observers it would not create a fundamentally new kind of color perception beyond the constraints of our three-cone vision.

So the main limit is not that displays are “digital” instead of “analogue”; it’s the fixed spectral output of the primaries and the gamut they can cover. RGB is used because it is a practical match to how humans see color.