The human visual system reduces every wavelength distribution to three numbers: the S, M, and L cone responses. Any two spectra that produce the same triplet are called metamers — physically different, but visually identical. This is why a computer monitor can reproduce any color with just three primaries. The monitor's light is nothing like sunlight, but it fools the three cones.
Some women carry a fourth cone type — a variant M or L opsin from a duplication on the X chromosome, with a peak sensitivity shifted by about 15 nanometers. Two stimuli that are metamers for trichromats (same S, M, L) may produce different responses in this fourth cone. Adjust the spectral stimulus below. The simulation finds a three-primary match that equates S, M, and L, then shows whether the two spectra diverge at the fourth channel.
The simulation models the fourth cone type as a Gaussian sensitivity curve peaking at 549 nm — between the M cone (534 nm) and L cone (563 nm). This is a plausible approximation for one common opsin variant, but the actual shift varies by individual. No one knows the exact sensitivity of any particular person's fourth cone type from outside.
More importantly: having a fourth cone type does not guarantee a fourth color channel. Most women who carry the variant opsin are non-functional tetrachromats — their visual cortex routes the fourth channel's output through the standard three opponent channels (red/green, blue/yellow, luminance), merging it rather than treating it as independent. Gabriele Jordan searched for functional tetrachromats for two decades; the first confirmed case (subject cDa29, 2010) required specially engineered stimuli — light mixes calibrated to be metameric for trichromats — because all commercially manufactured colors are already calibrated to fool three cones. The everyday world provides no test. Jordan had to design one. This simulation shows the test she needed to design, not what it would feel like to pass it.