How to scan an image (map) using only 256 colors?
You can use ColorSync for this job. It still requires you to scan the document with lots of colors, but you can reduce the colors to 256 using ColorSync (and do much more) by following these steps:


Open ColorSync: Applications->Utilities->ColorSync Utility
Open your scanned image: File->Open
Below the image select: Apply Profile->Named Colors->Websafe Colors 


I translated the names from german, thus they may be slightly different on an actual Mac OS X with english local setting.