On Mac if you install almost any version of the powerline, your MacOS Terminal.app would try to inprove contrast of text symbol, which is used as the end of the "bullet train":
This is somewhat documented in this answer on AskDifferent:
Terminal automatically applies a minimum contrast when displaying an ANSI (or extended 256-color table) color on the terminal background color, or when displaying the terminal foreground/text color on an ANSI background color.