To me, the difference between a "light" and a "dark" musk is basically a distinction between clean and dirty. White musk especially is very soapy and bright. A fragrances which feature it prominently are Helmut Lang, Les Nereides Musc de Samarkand and Body Shop White Musk. Dark musk is ambery and sweet, possibly animalic depending on the molecule or accord. Fragrances I consider to be dark musks are Kiehl's Musk and Coty Wild Musk.
China Musk is something I saw either at Body Shop or some other "hippie" store. It is somewhat like white musk in that the liquid is colorless, and the scent is clean. It has been a long time since I smelled china musk, so my recollection is rather vague.