Q: Aperture and El Capitan OSX

Indeed! I never noticed this, but I am seeing the same on my iMac with ElCapitan and Aperture 3.6. All thumbnails are upside down in list view.

My problem is that thumbnails appear correctly but not full-sized ones, which appear to be washed out (i.e. with too little contrast).

My problem is that thumbnails appear correctly but not full-sized ones, which appear to be washed out (i.e. with too little contrast).

That is a probably caused by the recent problem with "Onscreen Proofing".  Several forum members reported that turning on Onscreen Proofing in the View menu will make the photos appear washed out.

Not seeing that here:

.

Doing it here, too. Might be the first incompatibility (or maybe not the first, just the first one I've seen).

Correction: I am also seeing it here.

Plus, when I adjust the size of the thumbnails downward, they often change to a blue distorted view.

Another problem I've had with El Capitan and Aperture is that *sometimes* when I am brushing in enhancements (e.g. skin smoothing, sharpening, dodge, burn, etc) and I choose "Overlay colour" then the colour doesn't overlay.

count me in. me too!