[arch-general] URXVT background color

I run many urxvt windows on my screen. Some are for the host system and some are for my container (managed by systemd-nspawn). I am looking for a way to change color background according to hostname in order to quickly see which terminal I am on. My idea was to test $HOST variable in my ~/.xinitrc and give a specific .Xressource accordingly, but it doesn't work as .xinitrc is obviously not invoked when I fire a new terminal window. How can I manage what I am looking for (if I can)? Thank you for hints.