All the balls are the same color: brown

I would imagine it has to do with the retina where photoreceptors exist

/e all of the physical stuff has to do with magnification of the image (or incoming light) onto the retina

In the eye, cells only fire if the signal between the neighbors is great. There is also higher resolution concentrated at the center and drops off as you move away. Additionally the brain processes signals in context of the last signal it received. So if you put that all together, on a small screen the gaps are small enough that in the center of the eye, fovea, it will be processing more red than brown. You can also see the ball is actually not just brown. Its shaded which reduces the "fire near highly different neighbors" effect. The red though is solid.

There is also the fact that the eyes resolution drops off towards the edge. This makes the effect even harsher in the corner of your eye. As your eye moves from ball to ball your brain will be processing the new higher resolution versions based on the previous versions you saw in the corner of your eye. That effect fades away but your eye is actually always moving doing something called saccades, on a small screen with many prominent points it will do it a lot.

With a big screen alot of that goes away. The gaps are larger. The brown fades more slowly from light to dark so is percieved as a stronger color. Objects are larger so saccades will be less effective because they will be searching fewer objects rather than many in a tight screen. Lastly the lower resolutions at the edge of your eye will pick up more brown as well.

So there most definitely is physical aspects.

I believe this illusion is exploiting a quirk of human sensory vision so any machine learning will not be affected. It's the same property that most video encodings and compression algorithms exploit: Our colour and luminance perception does not have equal fidelity, i.e we dont sense as much detail in colour, where as we can sense more detail in luminance... in this case it means we can perceive the stripes, and given stripes alone we can just about discern their colour, but when mixed with a background colour it effectively blends the colour without blending the luminance.

I think this might be the same biological reason that we can't perceive much (or any) colour in low light, rods vs cones.

TL;DR our retinas are not CCDs they are a patchwork of two discrete components.

You make a good point, though different people perceive optical illusions differently. Remember the black/blue/gold/white dress? What colour should an AI call the dress when humans can't even decide? So it's actually even more complex than you might think.

This question also applies to audio illusions, such as the (in)famous Laurel/Yanny clip.

"Brown" is already going with "what humans would think", from the sensors that try to capture the image according to the peculiarities of human vision, to whatever representation the system has of the social construct of "brown".

You could even say that actually, the balls aren’t brown. They are made up of pure red, green and blue subpixels on your screen.

It’s really not a particularly interesting illusion.

Perhaps different people experience this image differently. For me, viewing the image on an iPhone, with no squinting at all, the balls look strikingly red, green and blue. It's only after zooming in on the image, that I convinced myself that the balls were brown with colored bars over them. As others have suggested, perhaps a small image size is what makes this illusion work.

Zoom out or move a little further from the screen and try again.

If you look at each ball one at a time, it's obvious that each is brown. The effect only works if you look at the balls with your peripheral vision and then make a summary judgement.

You may want to look into whether it's a learned phenomenon and also consider getting them tested for color blindness.

All perception is learned. Our sensory machinery just gives us noise. We learn through experience to differentiate the signal.

For example, paralyzed babies will also be blind, even if their bodies are moved by someone else.

Yup, my 5 year old saw them all as brown.

That's very interesting. But I think it probably is the fact they have fresher more youth receptors. If you think of when kids touch warm things and say it's too hot, it's a similar concept. Their eye receptor will fire more strongly reducing the main effects of the illusion. Although, if you are on a bigger screen, it may be that it's your eyes that are failing. Bigger screens or zoomed in this illusion breaks.

I wonder if that says more about their focal attention. If I really fixate on a ball, it becomes the same brown or tan color. I can scan from one ball to the next and mentally confirm that they all have the same color. While doing this, I actually feel my mind's "white balance" compensate as I shift my focus from one ball to another.

But, by default, I do not fixate on any ball. I take in the gestalt composition, and in this mode I perceive a field with three classes of balls with different tints. This happens as soon as I reframe my attention, even right after I have just sequentially confirmed their color as above.

There was also one interesting comment in the Twitter thread, posting a version that desaturated only the color stripe that overlaps the ball (at a bounding box rather than right at the circular edge of the ball). In this version, the gestalt composition shows me the same tint on all balls, and instead I see three classes of tinted tile in the background behind the balls.

Doesn't seem to be. I edited out all the lines with Photoshop, and what remained looked identical.