Tenacious Prussian
Tenacious Prussian (#1663FE) is a true azure with a neon character. It sits at the high-saturation edge of its family. Use it sparingly, as signage, accent, or highlight against darker surfaces. Its HSL profile (220°, 99%, 54%) places it in the highly saturated band at a mid lightness. Best used in small doses, like logos, CTAs, focus rings, or highlight text, where its saturation becomes a feature rather than noise. For a confident two-color system, pair it with its complementary amber. For something softer, pull in its analogous neighbors on either side of the wheel.
Etymology
Latin tenāx, holding-fast — adjectival suffix -ous. As a color modifier, tenacious implies a saturated-and-clinging quality where the hue grips its substrate with stubborn pigmentation. Sits at the bold-and-resilient end of the grid, parallel to unyielding and adamant in usage.
The first modern synthetic blue pigment — accidentally produced in 1704 by Berlin alchemist Johann Jacob Diesbach when contaminated potash turned a red dye unexpectedly blue. The result was Berlin blue (also Prussian blue): a saturated, slightly green-shifted deep blue with the matte finish of a pigment more lightfast than indigo and far cheaper than ultramarine. Cooler than cobalt, deeper than navy, with the art-historical weight of the pigment used in Hokusai's Great Wave.
Closest matches
The nearest named color in three reference sources, ranked by perceptual distance (ΔE76 in CIELAB). ΔE < 1 is imperceptible to most viewers; ΔE > 10 is clearly different. When two sources point to the same hex they’re merged into one tile; click any to open that color’s page.
Variations
Click any swatch to exploreHarmonies
Accessibility
How this color appears to viewers with the four major color-vision-deficiency types. Computed via the Machado (2009) physiologically-based model. If a tile matches the original, the color reads the same to that viewer.
The color used as foreground text against pure white and pure black, with the contrast ratio and WCAG 2.1 grade. Aim for AA (4.5:1) for body text and AA Large (3:1) for 18 pt+ headlines; AAA (7:1) is the gold standard for long-form reading surfaces.
Wide gamut
The CSS Color 4 wide-gamut form of this color. Both swatches render the same color on every display — the P3 form only diverges from sRGB when a designer pushes channels outside sRGB's reach.
This color is chromatic enough that authoring it as P3 native (instead of clamping to sRGB) gives a perceptibly more saturated render on wide-gamut displays — modern Macs, iPhones, iPads, and most recent OLED laptops.