While having fun with terminal colors, the old-school 16 colors, the extended 256 colors, and the more extended 24-bit colors, I noticed that the 256 color palette is modifiable with OSC codes (seems to work with st, xterm, rxvt, gnome-terminal; and NOT with at least putty and qterminal).
Palette changing does not sound very exciting when you also have 24-bit colours, but it does have its perks. Namely: changing the palette also affects all blocks already appearing on screen. Hence by side-loading palette control codes, you can change what is shown on the screen without actually touching the contents.
And that led to the incubation of a little utility that puts a clock on your terminal that updates even when the shell is idle, which then extended a bit with a 4-bar cpu meter. The resulting utility outputs a string with the appropriate placeholders (to be put into PS1: PS1="$(prompt)"). The output is escaped in bash style. The utility spawns a background process and periodically feeds palette changes to the tty.
Another utility abusing the same mechanism is nyancat. (Probably should rename, as nyancat is already a thing; but it is cat, and it does nyan, so whatchamacallit...). When fed the -g or -n flags, uses palette tricks in the same manner to make your eyes bleed with rainbows everywhere.
Also while pushing the latter one to git, wrote a small ANSI code "syntax hilighter" for gitea, the resulting snippet works well enough for my purposes.