Hello,
I have seen this in iPhoto when there was not the correct color space applied. It most offten happened with B&W photos. For me I brought the photos into Photoshop Elements changed the color space to sRGB, copied them to iPhoto they were then visible in iPhoto.
Though I did not have that issue with Aperture it is something to consider. Does this also occur in iPhoto, what color space is your camera set to? Does not occur in iPhoto? If you are on the latest version of Mac OS, iPhoto, and Aperture, the good news is they both, iPhoto and Aperture, use the same library. In the past they were separate. So, bringing photos successfully to iPhoto, close iPhoto then open Aperture those photos should appear the same as in iPhoto.
Also, you could do maintenance on your Aperture library. Close Aperture, hold both the Option and Command keys down, launch Aperture, when the dialog box appears release the keys run the maintence. For each one you will need to close then again relaunch Aperture holding down both keys, move to the next maintenance.
Cordially,
RicD