Q. Dear Oracle,
If I were to start a band called HotAct...would Coldplay sue me? Thanks.

They wouldn't sue but since you're here asking... why on earth would you pick such a dreadful name to represent you for all eternity? No offence but I'd go back to the drawing board and try to come up with something better.
I don't think you'd be taken seriously using that name either. However, if your band is a Coldplay tribute act, HotAct would be passable.