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<'bla>(...)
According to cat -v, <'bla> is preceded by ^[[38;5;10m, which, according to [1], "applies extended color value to the foreground", and further that 38;5;<s> sets "foreground color to <s> index in 88 or 256 color table".
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.
Then I don't know how it happens, because I only have yellowish background + black and red defined in my terminal (also white, which I don't see used anywhere). And here is a screenshot from my Terminal.app, where the lifetime is gray: <https://0x0.st/HqVq.32.png>.
I've checked and the 10th colour is set to #000000 in my Terminal.app settings.
If SGR 38;5;10 is not producing green, which it isn't, then it must be following your palette. My educated guess is that for some reason it believes that it should lighten the palette-specified colours from 8 to 15, and it's lightening your black to a very dark grey.
[1]: <https://learn.microsoft.com/en-us/windows/console/console-vi...>