The CMYK JPEG images created by ImageMagick are fine. They follow the JPEG standard. However, Adobe decided to invert the JPEG data in its files so some viewers follow the Adobe standard and others follow the JPEG specification. We try to follow the specifications as much as possible. To fix the problem, just add -negate to your command line to compensate for viewers that are broken. Note, its looks like Mac OS previewers has a bug and cannot properly display CMYK JPEG images. We tried negating the CMYK pixels and it showed the same wrong image view in both cases.