Don't know the cause.
Try force-generating new Previews:
- Select the Images that are incorrectly green in the Browser
- Hold down the "Option" key
- Run the command "Photos➞Generate Preview".
Did that fix the Previews?
Were these images fine before? Or, does this occur with a new import? Did you just upgrade the OS or Aperture? I would assume something must have changed somewhere.
What is your camera model (assuming these are RAW images), OS and Aperture version?
I've experienced the same problem... I'm not sure what generates the issue, but it appears that at some point the preview image becomes corrupted. Most of the time I'm able to fix the problem by clicking on the picture, while holding down both the control and options key.... The menu option "generate preview" will come up, and I can recreate the preview.
In a few instances, I've been unable to restore the preview. In this instance, I'll export the original to a folder on my desktop, delete the version in Aperture, and then re-import the original to Aperture...