Dual-contrast gamut

Backgrounds where both #000000 and #FFFFFF text clear the threshold. The two ratios always multiply to exactly 21, so the ceiling for a balanced pair is √21 ≈ 4.583:1 — at 4.5:1 the whole gamut is a shell 0.0083 wide in relative luminance.

Gamut map — hue × saturation

0° red120° green240° blue360°

in gamut, at the current band position no brightness of this hue/saturation can reach the band

Hue 0°

Saturation 0%

Position in luminance band 50%

0% sits at the dark edge (maximum contrast with white text, minimum with black); 100% is the reverse. At 4.5:1 the band is so thin the slider barely moves the color — at 3:1 it spans most of the mid-tones.

Preview

Sample text

Size (px)

Weight

#000000 on background

Handgloves

#FFFFFF on background

Handgloves

The 1px-stroke glyph is the 1.4.11 non-text case: at the 4.5:1 setting it clears 3:1 with room to spare in both directions.

Threshold

Contrast threshold both foregrounds must meet

Selected background

Hex — type one to test it

RGB

HSL

Contrast results for the selected background
Relative luminance
vs #000000
vs #FFFFFF
Body text 4.5:1
Large / non-text 3:1
AAA 7:1

Reference

Greys that pass both at 4.5:1: only #757575 (4.61 / 4.56) and #767676 (4.54 / 4.62). #747474 and #777777 both fail on one side.

Sanity check for any tool: ratio-to-white × ratio-to-black = 21 for every background, exactly. If a pair of reported numbers doesn't multiply to 21, one of them is wrong.

Blue ceiling: above roughly 63% saturation, no blue hue can reach the 4.5:1 band at any brightness — that's the hatched wedge on the map.