In the last two weeks I've begun a bunch of tangles on paper and card that had previously been marbled.
The shapes and colour distribution in the marbling suggest which patterns to use. A combination of black or white and two colours seems to work for me best. See my new Zentangle Bookmarks page for images!
Tangles that I feel suit marbled paper very well include Antidots, Betweed, Btl Joos, Daggerly and Droplets.