Before going nuts hunting down a camera/lcd/computer/software problem, it might be easier than that. The Canon 5DMIII has an LCD brightness setting in the menu system. By default, it tends to be brighter than what is actually captured in the scene because it is tuned for daylight shooting. I shoot night sports and I tend to turn my LCD down 1 to 2 clicks from middle because the middle setting tends to be brighter than the what the camera is recording. If you are going to use the LCD to judge exposure, you really need to take a little time to tune it so that what the LCD displays is what the camera is recording. Same general rule with using an external LCD. Use it for for focus and composition, but shy away from using it to dial in exposure and color.