I have used f.lux for a while now. Unfortunately, with the latest versions of Linux Mint and Ubuntu f.lux seems to be broken. Fortunately thanks to the answer on this Ask Ubuntu question. I found Redshift.
Installing Redshift was easy because it was in the default repositories for Linux Mint. I used Linux Mint's "Software Manager" app. Redshift is configured manually but using the example it's easy to do.
So if you're frustrated with f.lux being broken on your distro, try Redshift.