Discounting the entry for The War of the Worlds since it doesn't actually describe the act of colonization, I nominate The Struggle for Empire (1900) by Robert William Cole.
The planet Iosia, in an unnamed star system (that is not Sol or Sirius) is colonized first by Earth-humanity (in the name of the Anglo-Saxon Empire) and then in a hostile counter-...
1900: In The Struggle for Empire, by Robert William Cole, Iosia is a planet of an unnamed star somewhat more distant than Sirius:
For a long time past, a band of adventurers had been
colonizing a planet called Iosia, which was situated
far beyond the confines of the Sirian System.
The Struggle for Empire, Chapter III: The First Note of War
This planet is ...