I remember visiting Auschwitz (1) on a beautiful summer day. It's actually quite pretty. (That's assuming you filter out the gates, the wires, the interiors.) Hell is what was done there, not a cinematic filter.

Birkenau (Auschwitz 2), on the hand, was purpose built (Auschwitz 1 was a repurposed barracks) and that really does look like Hell, even on a nice day. Every last part of it screams "this was designed for obscene purposes".

DeOldify is going quite good job on old photos: https://github.com/jantic/DeOldify

I love all these colorizations, especially videos (easy to find on YT).

One thing I've been wondering about: Is it possible that the ML algo turns the faces into more "modern" faces because it's presumably trained on later color footage?

It's an inherent problem of all ML systems to extrapolate from training data only.

You can get dramatically different results depending on your training data set.

The only real issue with that is treating any of the outputs as "realistic" representations in terms of "close to the original scene". It's just a modern interpretation and the viewer needs to keep that in mind at all times.

To be fair to you, northern France, like the UK, will have many more grey and cloudy days than it will clear blue skies so in the main your imagination is probably right more often than not.

It does say that the film use wasn't very sensitive - so perhaps they could only take pictures on sunny days?

Autochrome was something like iso 10, even heavy overcast would be okayish for hand held photography

The article states that all the pictures required posing due to long exposure time, so I don't think "hand held" was an option. That being said, if you're gonna pose, you can just pose longer when it's cloudy.