[susam]

The 3-digit shorthand for hexadecimal RGB colours dates back to CSS1, if not earlier.

From <https://www.w3.org/TR/CSS1/>:

> The format of an RGB value in hexadecimal notation is a '#' immediately followed by either three or six hexadecimal characters. The three-digit RGB notation (#rgb) is converted into six-digit form (#rrggbb) by replicating digits, not by adding zeros. For example, #fb0 expands to #ffbb00. This makes sure that white (#ffffff) can be specified with the short notation (#fff) and removes any dependencies on the color depth of the display.

I'm quite fond of the 3-digit hexadecmial RGB notation. It's a concise way to express the colours I use for web pages or Emacs font locking. In these cases, I rarely need the full 16-million-colour range offered by 6 digits. The 3 digits are usually more than enough, at least to me.