Similarly to ninja, bazel captures stdout/stderr so the compiler's isatty() check returns false, disabling color. We need to explicitly opt back in to having color. Bazel is also good enough to strip color codes when its output isn't a terminal (tested with bazelisk build src/mongo/base:system_error_with_debug --copt='-fdiagnostics-color' | cat and an induced error) so it is safe to unconditionally enable this.