Aperture pro tip: disable 'Onscreen proofing' or scratch your head because your images look strange

Scratching my head is what I've been doing for a couple of days now (also cursing, a lot of cursing) because viewing images stored in my Aperture library looked strange; slightly desaturated, overexposed, etc.

I thought something was wrong with my library or with Aperture itself. I tried many things to get this working again, because no matter what others say, to me there's still no alternative to Aperture's image database features.

So, if you're having the same issue, or if you find that you don't see guides when trying to crop an image, make sure that 'View > Onscreen proofing' is disabled.