Aperture & Color Adjustment
The following is specific to 5D Mark III images, this does not happen on RAW images from other cameras:
I shoot mostly in RAW (the camera is set to "Faithful"), and this is what happens:
1/ I upload the image into Aperture
2/ Previews (thumbnails display normal colors)
3/ I click on an image, display a screen size
4/ Little sign on top of the image tells me "loading original"
5/ When loading is finished, the image is adjusted for what looks like contrast, and especially the reds pop out unnaturally - pink becomes bright magenta
6/ The reds don't go to their natural red or pink state
It seems that Aperture is picking up a profile from somewhere, or does something to the image while generating a preview.
It's the other way round. At first, you see the built-in preview that was created by the camera, using that 'Faithful' picture style. Aperture has its own RAW-engine, so it can't reproduce what the camera did and it can't use the same picture style. After the image is loaded, you will see the result of Aperture's own RAW engine and Aperture's default settings for that camera. That's the way it always works, with any camera, but apparently the settings you use for the 5DIII are much more different so it's much more noticeable for this camera.
What you can do is correct one image in Aperture, save this as an Aperture preset and then apply this preset to all 5DIII images upon import.