Why is there heavy dust and scratches only on the darker part of scanned color prints?

This can be normal when scanning pictures. The scanner works by shining a bright light onto the surface of the paper. Dust and fibers can easily reflect back into the scanning sensor, and are more easily visible against the dark contrast of darker areas of the image. Sometimes scratches or etchings on a paper may become more easily visible after scan than they were to the eye.

Try to carefully dust off your photos as best as possible with a soft brush, without being too abrasive. Also, try to carefully clean the scanner glass without leaving any glass cleaner residue behind. This should remove most of the major offenders of dust and fibers.

But, cleaning doesn't always get rid of everything. Some photo or scanning software have dust and scratch removal tools. You can also use photo editing software like Photoshop or Gimp to manually try to repair the photos.