How do I get systemctl to print in color when being interacted with from a non-tty?

watch -c  SYSTEMD_COLORS=1 systemctl status icinga2

man systemd says

   $SYSTEMD_COLORS
       Controls whether colorized output should be generated.

i.e., you can force color mode with that.

systemctl does not appear to have a mechanism for specifying when to color the output. A quick solution would be to shim isatty(3) to always return true, thus tricking systemctl into thinking stdout is interactive. Namely you could do:

# echo "int isatty(int fd) { return 1; }" | gcc -O2 -fpic -shared -ldl -o isatty.so -xc -
# LD_PRELOAD=./isatty.so watch -n300 --color systemctl status plexmediaserver

The -xc - at the end of the gcc command tells gcc to compile C code (-xc) from stdin (-). The rest of the flags tell gcc to create a shared object file named isatty.so. Note that this could very well break other programs which rely on isatty to return a legitimate value. It however appears to be fine for systemctl as isatty appears to be solely used for the purpose of determining if it should color its output.

Based on @KarlC's answer, here is a script which generates and then includes the library at runtime:

#!/bin/bash
set -euo pipefail

function clean_up {
  trap - EXIT # Restore default handler to avoid recursion
  [[ -e "${isatty_so:-}" ]] && rm "$isatty_so"
}
# shellcheck disable=2154 ## err is referenced but not assigned
trap 'err=$?; clean_up; exit $err' EXIT HUP INT TERM

isatty_so=$(mktemp --tmpdir "$(basename "$0")".XXXXX.isatty.so)
echo "int isatty(int fd) { return 1; }" \
  | gcc -O2 -fpic -shared -ldl -o "$isatty_so" -xc -
# Allow user to SH=/bin/zsh faketty mycommand
"${SH:-$SHELL}" -c 'eval $@' - LD_PRELOAD="$isatty_so" "$@"