How he got his eyes from black to green tho?

he was wearing contact lenses

Everything is in black and white (with touches of pink for blushes) until it flashes back to meeting Hwon as a boy (then you see the actual colour of his eyes for the first time - episode 16).

After the flashback episodes it stays full colour, but they don't actually show you Hwon's eyes (in the few episodes Hwon is in his eyes are hidden - see episode 26).

You first see Hwon's eyes in full colour (apart from flashbacks) in episode 32.

I guess the author hid them to make sure Hwon's identity stayed hidden until she wanted to reveal that he was the friend not the beggar.