Give this tool a hex value and it returns the nearest recognised colour name. That is a more slippery operation than it sounds, because a hex code identifies exactly one of 16.7 million possible colours while the English language has a few hundred colour words in common use. Every name is therefore an approximation, and the interesting question is which reference set you approximate against.
The named sets
CSS named colours. The 148 keywords a browser understands directly — rebeccapurple,
darkslategray, papayawhip. Inherited from the X11 window system by way of early HTML, they
are inconsistent, historically accidental, and full of oddities: there are four different
greys spelled two different ways, darkgray is lighter than gray, and the set clusters
heavily around a handful of hues while leaving others almost unnamed.
Larger curated lists. Collections running to several thousand names, drawn from paint catalogues, design references and crowd-sourced surveys. These give far better coverage and more evocative results, at the cost of any authority — nobody agrees on where “teal” ends.
How “nearest” is decided
Naively you could compare RGB values numerically, but that produces poor matches, because equal numeric distance in sRGB does not mean equal perceptual distance. Two colours 20 units apart in the blues look nearly identical; the same distance in the greens is obvious.
Matching in a perceptually uniform space instead — such as the one behind OKLCH — means “nearest” corresponds to what a person would actually judge as closest. It is the difference between a match that feels right and one that is merely arithmetically defensible.
What names are good for
Communication. “Make it a bit more teal” moves a conversation forward in a way “change it to #2a9d8f” does not.
Memorability and search. Naming a colour makes it findable and discussable later.
Sanity-checking. If you think you picked a navy and the tool says olive, something has gone wrong upstream.
What names are not good for
Specification. Never ship a colour by name between tools or teams. “Sage green” is a different value to everyone who reads it. The hex value is the specification; the name is commentary.
Design system tokens. Naming scale steps by appearance — light-blue, lighter-blue —
breaks the moment you need something in between. Number by lightness instead; the
colour scale generator covers the convention.
Assuming cultural universality. Colour vocabulary varies significantly between languages. Several do not lexically distinguish blue from green, and boundaries between named regions differ even among languages that do. A name that reads as precise in English may not survive translation.
Where people go wrong
Expecting an exact match. Unless your value happens to be one of the 148 CSS keywords, the name returned is the closest available, not a description. Small hex changes can flip the answer between two names that sound quite different.
Using CSS keywords in production palettes. They are convenient for prototypes and a poor foundation for a system — the set is uneven, and the names carry no ordering.
Reading meaning into the name. darkgray (#a9a9a9) being lighter than gray (#808080) is
the standard example. The names are historical, not systematic.
Once you have a colour you like, shades and tints will build its family, and contrast checker will tell you what you can safely put it next to.