This was mentioned in the novel, but it was the butler who choose yellow roses to signify breaking up, there's no way Hugo would have any tact about this at all. The butler choose yellow roses because in flower language, it can mean farewell and he wanted to soften the blow of basically getting a text message breakup.
no, I'm talking about the promise they made before marriage
Lucia said it was because if she started to cling onto him, she wanted to receive a sort of shock that would bring her back to her senses.
Damn I really don't like that condition about sending roses to say goodbye without even talking properly... like it's so half-hearted (imagine breaking up through email example from present days) and it may lead to misunderstandings (definitely). Also someone else can send them (that wench...) (〜￣△￣)〜