The overriding negative impact of imperialism was that the organic development of these countries was disrupted. Though China and Japan were not subjected to direct colonization by Europeans in the way India was, the governmental structures and ruling class of these countries were affected from the outside with consequences that were detrimental in both the short run and long run. Japan, due to Western influence, entered a rapid period of modernization aided by British and American military experts and soon became a power with ambitions of its own, including the attempt to create an Asian empire and to expel Europeans and Americans from all of Asia—with genocidal results to China, especially in Nanjing in 1937.
China in the aftermath of the Opium Wars became subservient to British interests and eventually entered a long period of disorder and political confusion, culminating in Civil War in the 1920s, war with Japan during the rule of the Kuomintang, and finally, Communism. It's impossible to disconnect these developments from the European and American interference that began in the nineteenth century.
In India, which first was subjected to the Raj of the British East India Company and then direct control by the Crown, there was not only the massive exploitation of natural resources, but from the start the large-scale mistreatment of the population, documented in the impeachment trial of the Company's Governor-General Warren Hastings. India had become a battleground for imperialist powers, the British and French, during the Seven Years' War. In the emergence of Britain as the sole occupier, a power-sharing arrangement was reached between the British and the indigenous princes who were allowed autonomous control over various provinces.
Nevertheless, this, in conjunction with the new spirit of religious and ethnic self-determination in the nineteenth century, exacerbated the fragile balance among the three major religions of India: Hinduism, Islam, and Sikhism. When independence was finally granted in 1947, the division of the Subcontinent into India and Pakistan resulted in a huge movement of populations and massive fratricidal violence. In the 1970s, the secession of Bangladesh from East Pakistan had similar results, including genocide. None of these developments would have occurred in the absence of the legacy of colonialism.
In history, one can rarely find large-scale events that are totally, absolutely good or bad, and one never knows if the history of the world might have been even worse if things had not happened as they did. Imperialism, despite its fact of having been a negative, exploitative, and oppressive phenomenon, can be said to have created a truly global culture. In India, the British legacy has been the use of English as a lingua franca; people from the Subcontinent can immigrate to the UK, the US, and the rest of the English-speaking world, obtain jobs, and bond with those of different backgrounds. In China and Japan, the positive legacy of Western influence is chiefly economic; both countries have become industrial powerhouses. Yet, there is of course no way of knowing if these, or similar developments, would have happened absent imperialism—or of knowing whatever other things would have happened for good or ill if the events of the past 250 years (and longer) had been different.