I would say simply שבת שלום (Yes, I know, Shabbat is only part of the weekend, but still it's said many times before the weekend. I say it to my co-workers on Thursday, the last workday of the week.)
At the end of a workday -- even if it was 4 o'clock on a sunny summer afternoon -- on departing from work I would say "Good night" to my coworkers, or they would say "Good night" to me. I have also had occasion to arrive for, and to depart from, work assignments at 0300 hours.
this is one of those things that different people interpret differently. I use "afternoon" to refer to the time after 12 pm (noon) and before the end of the customary workday (5 pm). I don't adjust my usage based on daylight hours.