Ok, now this is going to be a tough one: for the record, I do not in any way support colonialism and the evils it caused, I would never dream of trying to justify its crimes. This is only out of sheer curiosity and nothing more.
On to the actual challenge: maybe I am completely wrong, but it seems to me that many colonial powers, well, they didn't always manage their colonies effectively. In this, sadly I don't mean effective for the natives, as few cared about them, but for themselves, in other words, oftentimes, they shot themselves in the foot or they implemented policies that would later come back to haunt them. Your challenge here is this: have colonialism be as "effective" and as "successful" as possible, with a PoD after 1900. That could mean more profits for the metropole, more "cultural imperialism"(think France and the Francafrique perhaps, or the way Britain spread English around the world) or simple expansion.
I guess some useful ideas could be no World Wars, so that more metropolis have more people to send to their colonies. Another idea could be that in the Second World War, the British fare better against the Japanese in the Far East and thus the myth of the undefeated Europeans isn't shattered. Maybe for some reason the Americas are in turmoil and Britain cannot have its informal Empire in Latin America, while at the same time many immigrants are forced to flock to the colonies to seek opportunities instead of the Americas.