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.

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

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.

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.

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.