If gtk-redshift crashes at Unity's startup because no network connection is available, you can start it with this script instead, to keep trying every 10 seconds until it succeeds.

gtk-redshift.sh

#!/usr/bin/env bash
while true; do
  pgrep redshift && exit  # Already running.
  gtk-redshift
  # Died? Check again after 10 seconds.
  sleep 10
done