The use of the colon is correct. It's acting as an introductory address similar to To Whom It May Concern:.
I'm guessing you want to write something on reddit. It's the only place I know where they talk about a sub (sub-reddit). I would write it like this:
- To all the Leo Messi fans here in this sub: if he joined either City or Chelsea, would you then hate him?
Note the declaration "To all..." Also, you would not use quotes because you're the one talking, and you don't quote yourself!
Assuming this is your purpose, slight changes in phrasing may be preferred to convey different meanings. Let's say Leo Messi fans love good old Leo, and hate all the players on the other team. And you want to point out the absurdity of hating someone just because they are on a team. Then I would say,
- To all the Leo Messi fans here in this sub: would you suddenly hate him if he joined either City or Chelsea?
On the other hand, let's say he's thinking about moving, and you want to know if his fans will stop being his fans merely because he's playing for another rival team. Then I would say,
- To all the Leo Messi fans here in this sub: are you going to hate him if he does join City or Chelsea?