When looking at places like Africa and the Americas it was justified with them taking on the mantle of leadership and helping "those who can't govern themselves and bringing the light of civilization to heathens and savages" or something along those lines. How then can they say that about China?
China was never effectively conquered and colonized by the European powers in the manner that Africa was. The extent of European colonization of China were treaty ports such as Hong Kong or Port Arthur which were established to open up trade between Europe and China. The reason European powers did not colonize China in the late 1800's were almost entirely economic as it was for more profitable to simple trade with China than to establish an empire there which would have been incredible expensive to maintain. A good example of this relationship was the opium trade between Britain and China where Britain would import opium into China in exchange for a wide variety of goods. Additionally, Europe did not consider China a "very civilized" nation due to the lack of industry and for cultural reasons such as the practice of foot binding.
You may be able to find a more complete answer than mine in this thread as your question was asked a few years back.