My first thought was that conquest was the immediate action and that colonization took place over a longer time span. Sort've like invasion and occupation. I'm not sure though that conquest wouldn't also apply to uninhabited lands. The Old World explorers thought of themselves as conquerors AND managed to convince themselves that the lands were uninhabited because they found only "savages." And this profile of Sir Edmund Hillary describes him as the "conqueror of the North Pole" even though it certainly wasn't colonized.

Not criticizing, just word-playing.