This German article is about a unique collection photographs Hitler made of artwork, to keep if safe for the future:
http://morgenpost.berlin1.de/content...on/786804.html

Now, that is nothing new you'd say. I agree.
But it does have an interesting note for us.
They used new (Agfa) colour photo techniques to capture the artwork as good as possible. But they held two big meetings in an attempt to solve one big problem: there were great difficulties to get the colours right...

I think that I have nothing to add to that to make you understand how carefull we must be with original colour photo's....
And that added to the influences time of day (morning and evening have a warmer, more reddish light then noon), shades, weather and clouds.