A video on the conservation of the mosaics at Zeugma, Turkey. Go to 09:29 and it mentions the sinopia, painted patterns on the setting bed.
Hopefully I can find out how to access the reports on this (please let me know if you know where they are published) to see exactly what they found. http://www.youtube.com/watch?v=QUJ7PHCNOVs
I'm not sure if this has been posted before but here is the report on the sinopia found in the Lod mosaic, http://www.cut.ac.cy/euromed2012proceedings/shortPapers/203.pdf