How can I extract map data from map images with color overlays?

All my local institutions publish a lot of data as map images with color overlays. Is there a way to extract that data into an actual map? Bonus points if it can be done with open source tools.

Examples: * sound-map of the city - every 10+ years they publish high resolution pdfs with images that contain different sound-zones, I would like to have that on an actual map, so I can easily check addresses and other things.