To get a proper war - rather than just some diplomatic squabbling or a localised conflict - you need a chain of events that goes right up to the Great Powers without being able to be defused along the way. Something in the Balkans is good because it can involve Serbia, which brings in Russia and AH, which brings in Germany who has to go west because that's what the plan say, and that means France and possibly Britain come in. And something Balkan is also small enough, at least initially, for people not to fully realise what they're starting - they can hope that, for example, Russia will back down on Serbia.
Another good one would be picking apart the Ottoman Empire. This is probably another Balkans route though, give the potential for squabbling over Constantinople. Ah, there's just too much potential for trouble in the Balkans.
In contrast I find it harder to see a full war developing from some colonial squabbles. There's just more separation between the colonies and Europe, and less of an obvious path for escalation.