Colonization is not just forcing culture or language. The material basis of colonization is imperialism i.e. forcing an unequal exchange between the colonized and the colonizers. There is no unequal exchange between Tibet and China as they are the same country. Capital and labour flows freely in and out, with the majority of the Tibetan population living outside Tibet. The majority of the Tibetan population has a positive opinion of the Chinese government, the same can’t be said of any colony. And on top of that the Tibetans are free to practice their culture(except the feudalism, pedophilia, and torture part of course).