"宗主国的公民" can be translated into English as "citizens of the sovereign/metropolitan state" or "subjects of the ruling/dominant power" (though "subjects" is somewhat more archaic and can imply a feudal or colonial relationship where people are under the authority of a monarch or ruling power).
In modern contexts, especially when referring to colonial history, "citizens of the colonial power" or more specifically, if the context is clear, "citizens of [the specific colonial/sovereign state's name]" (e.g., "citizens of Britain" for a former British colony) would be more commonly used and precise.
For a general and neutral term, "citizens of the sovereign state" is a good choice.