Hex color

A hex colour is three numbers in a trench coat. The six digit form is three pairs, red then green then blue, each pair a byte written in base 16 and therefore running from 00 to FF, which is 0 to 255. #4F46E5 is red 79, green 70, blue 229. Nothing about the notation is a colour model of its own: it is sRGB, the same coordinates rgb(79 70 229) names, in a spelling that fits in one token and survives being pasted into a chat window.

The short forms are the ones that trip people. Three digits expand by doubling each digit, so #f0c is #ff00cc and not #f00c00, which is why only a lucky subset of colours can be written short. Four and eight digit forms append alpha in the same way: #4F46E5CC is that indigo at 80 percent opacity, and #f0c8 is #ff00cc88. Case does not matter to a parser, though a codebase that mixes #FFF and #fff will fail its own diff review eventually. Alpha in the value is worth watching, because a colour carrying its own transparency is a different thing to reason about than a solid one sitting under an opacity property.

What hex is bad at is arithmetic. Reading #4F46E5 tells you almost nothing about how light the colour is, and there is no digit you can nudge to make it a step darker without also swinging the hue. That is the whole argument for authoring in OKLCH and for building a colour ramp in a space where the numbers mean something, then letting the build step emit hex for the places that still need it. Hex is also capped at sRGB: a colour outside that gamut has no hex spelling at all, so a notation that once described every colour a screen could show now describes only the older ones.

The staying power is social rather than technical. Hex is what a design tool puts on the clipboard, what a brand guideline prints, and what someone reads out loud in a meeting, so it remains the handoff format even for teams whose tokens are authored somewhere else. Treat it as an output format, not as the place decisions get made.