Judd (1968) and Silberstein (1938) are the earliest identified sources framing color-difference space as intrinsically curved, per Bujack et al.'s (2025) own bibliography

Bujack, Stark, Turton, Miller & Rogers, "The Geometry of Color in the Light of a Non-Riemannian Space" (Computer Graphics Forum 44(3), e70136, 2025) — a Los Alamos National Laboratory paper unrelated to Ahrens et al. 2024 and not itself citing it — lists two much earlier works in its own bibliography, read directly in full via extract_pdf: Deane B. Judd's "Ideal color space: Curvature of color space and its implications for industrial color tolerances" (Palette 29, 1968) and Ludwik Silberstein's "Investigations on the intrinsic properties of the color domain" (JOSA 28(3), 1938).

Judd's title states the curvature claim as its own subject outright, making him — on the evidence assembled so far — the earliest clearly identified figure to take the curvature of color-difference space as an explicit subject, predating Urban et al. 2007 by nearly four decades and Ahrens et al. 2024 by over half a century. (Differential-geometric framings of color space as such are older still and sit in the same bibliography: Bujack et al. build on Helmholtz's 1867 idea of deriving the color attributes from a Riemannian perceptual metric, formalized by Schrödinger in 1920 — but assuming a Riemannian metric is not yet asserting the space is curved.) Bujack et al. cite Silberstein, thirty years earlier still, for the distinction between straight lines and shortest (geodesic) paths in color space: "The discrimination of straight lines from shortest paths is in accordance with the one by Silberstein [Sil38]." That distinction is suggestive of, but does not by itself entail, nonzero intrinsic curvature — the "straight" lines are the affine mixing lines, and a metric whose geodesics differ from them can still be flat — so what Silberstein himself concluded about curvature awaits a direct read.

This narrows a common but imprecise framing in the vault: the "Gaussian curvature of color space" claim is often introduced as if it originates with modern computational work (Ahrens 2024, or even Urban 2007), when the geometric intuition itself is a mid-20th-century result in classical colorimetry.