It was based on ideas that led to the comic book.
(As a friend told me) apparently when they were creating the script for the movie, Alan Moore was
still working out the basic design he wanted for the comic book.
So the movie makers had to add in elements to fill in the blank spots in their plot. Which is why
some characters are different, and some are completely new, and why the story goes in a somewhat
different direction.
The movie was color all the way thru, but it was meant to have an "old world" feel to it, so they
used very subdued coloration for most of the sets.
I liked the movie, the only problem I saw was they had only two hours to explain who all the members
of the league were, and their motivations and abilities, before throwing you into the main story.
While the books could take more time (whole chapters in fact) to flesh out each character, and tell
a story as they go along.