They are colours that have names in CSS. You can access 16M colours with #RRGGBB syntax but these colours are aliases to specific values and can be specified by name.
I remember, way back when some of hadn't been born and the web was new and exciting, noticing the difference in greys when using the same colour names in a desktop tool's output and it's HTML documentation.
I actually think I developed a preference for some colours and combinations based on using HyperNeWS, which had a particularly tasteful set of colour combinations (at least for 1990 or so!).
https://developer.mozilla.org/en-US/docs/Web/CSS/named-color