
If you answered 'red' to this question, you are of course....both entirely right and wrong, at the same time!
The photo was taken by Akiyoshi Kitaoka and I came across the story in David McRaney's really interesting book: How Minds Change - though you can also see and read a bit about it here on his website.
So, how were you both right and wrong?
Well, you were right because we all know that strawberries are red and the original strawberries used for this photo were just as red as you would expect.
But you were also wrong, because the photo really isn't red, and thanks to the wonders of modern technology, we can measure this. I copied the image into MS Paint and used the dropper there to analyse the red, green and blue mix for various spots. They all showed something similar to what I've shown here:

Every point I selected had equal quantities of blue and green and significantly less red. Green and blue together create what physics textbooks usually call cyan - which is pretty much the colour of the surrounding to the strawberries above. Add in a little red and it should look something like the highlighted point on the colour scale opposite: a sort of cyan-grey mix.
So why does it look red to most of us? Well, McRaney explains that the odd lighting in the photo - the washed out, over-exposed look, which was deliberate - serves as a visual clue to us that something is amiss, so we mentally correct for this and ramp up the red in the image.
But what's really interesting to me here is that this is not something that we can switch on and off. Its not like the famous duck/rabbit image below - which we can train ourselves to see as either animal. I've read all about this, analysed the colour mix in the photo and looked at the image dozens of times - and I still see those strawberries as red.
This all, of course, connects up with the famous photo of 'the dress'. And the explanation for both is pretty much the same.
And the illusion (if that is what we should call it) may have an evolutionary link: the ability to filter out blue light - and mentally correct what we are seeing - may have helped our ancestors as they studied the landscape for either predators or prey.

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