
Spoils!
I felt so bad for green. I'm not sure what was accidentally said that got him rosebushed. But I went back to read his first appearance and he was such a sweetie ;_; .
Also,due to green, I have a theory that White is still alive and that he traded places with yellow at some point. "White" turned into yellow roses when he passed, while his rose color was very specifically shown as white previously.
If the "gift" that White requested from Mr. Rose was hair dye.....
Also, I suspect purple is the murderer due to the previous deal he had with Mr. Rose.
Anyways. These are just my guesses, but gods, I love this story.

Doyoung says something like “So what if Green Hyung is fake??” And Mr. Rose decides that’s good enough and kills green hyung.
You are so smart I didn’t even think about the color of the roses I bet you’re right that he asked for hair dye. Either purple killed him to make Mr. Rose happy or white hyung dyed yellow hyung’s hair then killed him to frame cast hyung.
I love this story too so underrated.
Spoilers for chapter 29&30 of the raws
White haired hyung and green haired hyung are both discovered as fakes and they turn into rose pedals. Green haired hyung was the one who came up to cast hyung near the beginning of the story. Cast hyung is kind of like on trial for the death of white haired hyung.