How to scan an image (map) using only 256 colors? Is there a way to can an image on OSX using only 256 colors rather than millions or billions?

I am scanning maps which only use a few colors, so I think I want to keep the color depth low so there is clear separation between lines and background (kind of like B&amp;W vs. Gray Scale). Note I want to do this at scan time to improve the scan.

I am on 10.7.4 with a HP C4599. HP Scan doesn't let change the color depth at all. Preview (or Image Capture) only let's me choose between millions and billions.
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.