There are not many ways to say these greetings in Korean, since everyone just says "Hi" all day long.
I will do my best to find some phrases for you though, haha.
- Formal: 좋은 아침입니다/좋은 오후에요.
- Informal: 안녕! or 좋은 아침!
Just saying 안녕 will be okay, but if you really want to say good afternoon....
- Formal: 좋은 오후입니다/좋은 오후에요.
- Informal: 안녕!
Good evening/Good night
- Formal: 좋은 밤입니다/좋은 밤이에요.
- Informal: you know where this is going... 안녕!
I hoep it helps :)