Another thing, which is probably more important, is the fact that Synder decided to use grey in the mask, thus missing the point.
Which would really be necessary in order to make it look like an on-stage effect rather than CG - in fact, clearly demarcated black-and-white would probably read as cel animation no matter what they did with "lighting" it.
Or maybe Rorschach describing the black and white as "not mixing, no grey" is what Moore really meant by the novel being unfilmable.

Rorschach blots are of varying density on the cardboard, of course. Lotta grey.
http://www.psychblog.co.uk/wp-content/uploads/rorschach_blot_06.jpg