"宗主国的公民" can be translated into English as "citizens of the sovereign/metropolitan state" or "subjects of the sovereign power" (the latter is a more historical or formal term).
In modern contexts, "citizens of the sovereign/metropolitan state" is more commonly used, where "sovereign state" refers to the dominant or ruling country in a colonial or imperial relationship, and "metropolitan state" specifically refers to the mother country in a colonial context.
For example:
The citizens of the sovereign state enjoyed certain privileges not available to the colonized peoples. （宗主国的公民享有殖民地人民所没有的某些特权。）
As citizens of the metropolitan state, they had certain obligations and rights. （作为宗主国的公民，他们有一定的义务和权利。）