What happens when you zoom in to 1:1 on those images? This behavior can generally mean one of two things. One, your monitor profile is bad. This often happens on windows machines after a video card driver update. The solution is recalibration. Second possibility is that your previews are somehow corrupt. This can be checked by zooming one-to-one. If the image looks correct, the previews for those images are bad somehow. Usually this can be fixed by generating new previews on those.