I'd have thought that you designated which balloon you were hooking on to and then followed its fate. But the facts that the hook is resolved last and that there is no commentary about how/when/whether you designate which color you're hooking up with (assuming that more than one start out a single level above you) make me wonder if that's really how it works. Is it that you use the grappling hook in anticipation of the next turn -- i.e. you wait til the end of the turn you've played the hook card to see who ends up one level above and then hitch your balloon to that one for the next round? (In which case, does what you play the next round matter at all?) Is there some other possibility I'm overlooking?
The player who uses a grappling hook chooses which balloon to grapple to (if there is a choice) after seeing what card is played. Only balloons that played Ballast cards or Super Motors can be grappled, and then only if no sleeping gas was played by a balloon one level higher.
The rules say
Hooks can be used to hold onto balloons with Super Motor or Ballast Cards, it is the player's choice. A Sleeping Gas will break a Grapple.