Luminance carries the sense of order

What it is

Ask readers "which of these two colours means the larger value," and they mostly check lightness: brighter or darker (per the palette's declared direction). Hue sequences (red → orange → yellow) yield no stable sense of order — judgements of red's "intensity" vary by person and context; saturation assists (more vivid = stronger) but holds only within one hue and spans a narrow range. The design discipline for sequential palettes follows: run the main gradient on lightness (low values dark, high light, or reversed but consistent throughout), with hue and saturation as accompaniment (lightness ramps within a single hue, or low-saturation two-hue gradients) — that is what secures readable order. It also explains why palettes sorted by hue leave readers without an order to read.

Why it happens

Lightness is the visual system's oldest intensity dimension: the dark-to-bright mapping predates colour in perception (the luminance channel sits earlier in the pathway than colour-opponent channels), and "brighter = more/stronger" comes preinstalled; hue has no such total order — hue is a ring (cyclic), rings carry no agreed "weak to strong" direction, and readers' ordering falls back to personal associations ("red hotter than blue"), unstable by nature. Lightness also has the best discrimination threshold: adjacent lightness steps within a hue resolve at smaller differences than hue or saturation steps, laying more distinguishable tiers into the value range — and tier count is precisely the resolution sequential encoding needs.

Where it stops holding

Lightness-as-carrier presupposes display fidelity: washed-out projectors, cheap screens crushing shadows, and print shifts all damage the lightness gradient first (hence the greyscale test's value — colour-deficiency group); dark modes need reversed strategies (the "brighter = larger" reading flips psychologically on black backgrounds; re-tune direction per medium). Low-stakes display contexts (data art, children's science) may let hue participate in order, accepting readability loss with numeric labels attached. Each side of a diverging palette is its own lightness gradient; midpoint meaning belongs to the diverging group.

Applying it

• Palette construction rules: single-hue lightness ramp (deep blue → pale blue) or low-saturation two-hue gradient, monotone in lightness (first leaf), spanning at least L* 60; hue participating in no more than two stops.
• Fix one palette direction site-wide (dark = low), legend arrows matching; dark themes get separately built mirror palettes.
• Verification: desaturate to greyscale and ask readers to order five stops of the ramp; correct ordering surviving greyscale means lightness carries the order and hue has not usurped it.

Related

• Same group: U4.02.1 Sequential palettes must increase monotonically in perception · U4.02.2 Rainbow palettes create false boundaries
• Nearby: U4.04.2 Separation must survive through luminance differences · U1.04.1 Reading characteristics of the lightness channel
• Search terms: luminance ordering · sequential ramp · greyscale test · lightness gradient