Sequential palettes: encoding ordered data
Sequential palettes encode data with a natural low-to-high order — temperature, revenue, time, density. The principle: lighter values represent lower quantities, darker values represent higher quantities. Single-hue sequential palettes (light blue to dark blue) are the most reliable and the most colorblind-safe. Two-hue sequential palettes (yellow to blue, yellow to green) can provide more perceptual range but must still maintain a consistent lightness progression — the hue transition must not create a local lightness anomaly. The test: convert the sequential palette to grayscale. Each step should be visibly darker than the last. If any step appears lighter than its neighbor in grayscale, the palette has a lightness inversion that will produce ordering errors. OKLCH is the most effective color space for building sequential palettes because it provides perceptually uniform lightness — a 10-unit OKLCH lightness step looks the same regardless of hue. Building a sequential ramp in HSL or RGB often produces lightness anomalies at certain hues (yellow is perceptually much lighter than blue at the same HSL lightness).