The Tribute System: Understanding East Asian Diplomacy
In pre-modern East Asia, China's neighbors sent regular missions to its capital to ‘pay tribute’ in the form of various goods, and in return China bestowed gifts and invested titles upon their rulers. The tribute system dates back to the ancient state of Zhou, and by the Tang dynasty it had become established as the normal order of foreign relations in East Asia, and remained so for a millennium. Hence it might appear that all China's neighbors were subject nations, but this is not necessarily so. Though Britain and other European nations traded with China in the 19th century based on the tribute system, they had never been seen as subjects.
For smaller countries in East Asia, paying tribute to China was a foreign relations policy that prevented Chinese invasion and meant inclusion in a Sino-centric world order that enabled lucrative trade and valuable cultural exchange that included upgrades of technologies.
In economic terms, the tribute system was an important channel for international trade. To the states paying tribute, China often granted gifts worth many times as much the tribute received, and in this way trading took place. A large volume of private trade also took place through the merchants who accompanied tribute missions. Thus the tribute system continued until the early 19th century as China's major form of foreign relations and included Korea and all neighboring countries.
While pre-modern Korea did indeed adhere to the tribute and investiture system, this simply meant that Korea was part of the foreign relations order in East Asia, not that it was a subject or colony of China in any effective or nominal sense. Joseon controlled all its own domestic affairs.
Through the tribute system, Joseon maintained strong ties with the Ming dynasty, but when the Manchus defeated Ming and established the Qing dynasty, Joseon began to look down on China as a land ruled by barbarians. Though forced to pay tribute after losing two wars with Qing, Joseon maintained the belief that Korea's culture was superior to that of China.
It is notable that despite sharing borders with China for thousands of years Korea has managed to remain independent. To see Korea as a subject of China because of the tribute and investiture system is to ignore this fact.