I think the makers of the Colour of Magic movie were afraid to be dubbed racist had they included a silly oriental character like Twoflower. The same thing happened when an American version of Red Dwarf was made; Lister, who's hispanic in the UK version, was a caucasian American in the US version.
Personally, I don't like the fact they've made Twoflower American, since it doesn't fit with Interesting Times. Now, if they ever make a movie of Interesting Times, they have to either change the characters from the Color of Magic movie, make Twoflower a tourist on the Counterweight Continent as well, or make a parody of China and Japan with caucasian Americans playing all the Chinese and Japanese.

