Hex color
The #rrggbb code: six hexadecimal digits giving the red, green, and blue amounts of a color, from 00 to ff each.
See it
What it is
A hex color is an RGB color written in base 16: '#rrggbb', where each pair of characters is one channel from 00 (none) to ff (full). #ff0000 is all red, #000000 is black, #ffffff is white, and anything with three equal pairs (#888888) is a pure gray. The 3-character shorthand just doubles each digit, so #f0c means #ff00cc, and a 4th pair adds alpha: #00000080 is black at roughly 50 percent opacity.
It survives because it is compact, copy-pasteable, and universally understood: design tools, CSS, Figma, and every AI tool you prompt will take a hex code without argument. Handing a model a specific hex is the fastest way to stop it from inventing its own blue.
Gotcha: hex is hard-wired to sRGB, so it cannot express the punchier reds and greens of a wide-gamut Display P3 screen, and the digits are not perceptual. #808080 is not the color halfway between black and white to your eye, and you cannot eyeball 'a bit darker' by nudging digits. For building ramps or mixing colors, work in OKLCH and export to hex at the end.
Ask AI for it
Use exact hex codes for every color in this design instead of named colors or vague descriptions. Set the background to #0b0b0f, body text to #e6e6ea, the primary action to #4f46e5 with #ffffff text on it, and borders to #2a2a33. Include 8-digit hex with alpha (like #ffffff14) for any translucent overlays, and list every hex value used in a legend at the end.