Ninja captures and buffers stdout and stderr from builds (except when pool=console) which tricks the istty check in compilers. The recommendation from https://github.com/ninja-build/ninja/wiki/FAQ is to add -fdiagnostics-color to the compile flags. Ninja adds its own istty checks and will filter out the color codes if the output isn't a terminal, so it isn't harmful to add the flag always, even when ninja output is redirected.