Earlier quoted context omitted.
I don't see it here, but one example would be help messages saying "consider introducing lifetime `'bla` here", which are preceded with a source code quote saying something like: fn foo (...) According to cat -v , is preceded by ^[[38;5;10m, which, according to [1], "applies extended color value to the foreground", and further that 38;5; sets "foreground color to index in 88 or 256 color table". [1]: https://learn.mi…
Oh dear. You will I hope not be too embarrassed by learning that the 10 in SGR 38;5;10 (that control sequence) is colour index 10, which is less than 16, and a member of the very AIXTerm 16 colour set that you want. Thus the rust compiler is not in fact using a colour outwith the 16 AIXTerm colours.
I've checked and the 10th colour is set to #000000 in my Terminal.app settings.