Google informal letter and you ll see loads of hi..
About your previous message, yes I agree there s a special tone.
Butin semi formal letter to your colleague you won't write what's up dude. As well as dear Mr. Switch - too formal.
For this task you should use something in between, more relaxed tone.
As I wrote before, there's no info wherever it was your colleague who was your best friend from school times.
I'm certain that for marks 5.0-6.0 we can forget about semi formal.
But for those who want 7.0 and more it's crucial to be able to write semi formal.