[follower]

I'm guessing it may have been one or both of these `xterm`-related documents:

* <https://invisible-island.net/xterm/ctlseqs/ctlseqs.html#h4-F...>

* <https://invisible-island.net/xterm/xterm.faq.html#color_by_n...>

More details in my other comment: https://news.ycombinator.com/item?id=35157058