I'm getting interested in these as a category.
Examples: the Japanese in Micronesia (1915-1945); the Germans in Africa (1880s -- WWI); the Italians in Libya (1912-1942).
Disqualified: various occupations during and around the World Wars that were never generally recognized. So, for instance, Japan was running Manchuria 1931-45, and Germany had Poland 1939-44, but those were extralegal occupations so I'm leaving them out.
Also disqualified: when one sort of colonial empire segues into another one under the same colonial master. So, the Belgian Free State (1884-1908) just turned into the Belgian Congo, and various African colonies went from being League of Nations Mandates to United Nations Trust Territories without much change.
So why are these interesting?