Don't quickly knock Trump for this one. This is one of those disputed things in history. Korea says never, but there are many references to Korea once being a colony or tributary state of the Quing Destiny Empire. It depends on how you look at it. The modern nation of Korea never has. However, if you go back to the 1600s, part of Korea was a colony, albeit a warring one.
Being a tributary state doesn't mean they were part of China, it means the opposite actually. China didn't have to invade and conquer itself.
Korea, even long before they were Korea was a separate nation, at some point they were 3 or more nations on the peninsula. They might have sent tribute to China (or Manchuria) but that is very different than being a part of it. They had their own kings.